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Volume 9, Number 2 begins with the first half of an interview with French mathematician Alice Guionnet. Prof. Guionnet is a professor of mathematics at MIT and an expert on random matrices.

In addition to the interview, there are 2 more articles in this issue that pertain to probability and statistics. One is the concluding half of Prof. Elizabeth Meckes articles on the laws of probability. This time, she pulls up the curtain on the central limit theorem. The other is this issue’s Math In Your World, which describes an activity led by Girls’ Angle Support Network visitor Jinger Zhao. Jinger is a financial modeler who works at TwoSigma, a hedge fund based in New York City. At the club, Jinger uses statistics to model the connection between wingspan and height.

Anna continues her investigation of irreducible polynomials over the finite field with 2 elements. In this installment, she works out the roots of all the irreducible polynomials of degrees 4 and 5. Anna’s entire investigation traces back to an exercise suggested by Prof. Judy Walker in her interview from Volume 8, Number 6. Do you think you can see where Anna might be headed? If you do, follow your thoughts and see where they lead. You’re invited to tell us about it; we’d love to hear from you. If you’re falling in love with polynomials over , check out this proposal for a new PolyMath project and the comments that follow.

Multiplication and exponentials are fundamental concepts in mathematics. For anyone working on learning these concepts, we hope Addie Summer’s *Thoughts on Multiplication* and our Learn by Doing on exponentials will be of use. The cover, which was created using MATLAB by MathWorks, honors multiplication.

Emily and Jasmine continue their quest for “nice” triangles. This time, they explore integer-sided triangles that have a 120 degree angle and establish a beautiful bridge between these and integer-sided triangles with a 60 degree angle.

We conclude with some notes from the club. If you’re a girl, aged roughly 10-18 in the Greater Boston Area, you’re welcome to join. Our next session begins January 28, 2016.

We hope you enjoy the Bulletin!

Finally, a reminder: when you subscribe to the Girls’ Angle Bulletin, you’re not just getting a subscription to a magazine. You are also gaining access to the Girls’ Angle mentors. We urge all subscribers and members to write us with your math questions or anything else in the Bulletin or having to do with mathematics in general. We will respond. We want you to get active and *do* mathematics. Parts of the Bulletin are written to induce you to wonder and respond with more questions. Don’t let those questions fade away and become forgotten. Send them to us!

We continue to encourage people to subscribe to our print version, so we have removed some content from the electronic version. Subscriptions are a great way to support Girls’ Angle while getting something concrete back in return. We hope you subscribe!

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Volume 9, Number 1 features material from two mathematicians: an extensive interview with Elizabeth Munch, Assistant Professor of Mathematics at the University of Albany and the first of a two-part article on the laws of probability by Elizabeth Meckes, Associate Professor of Mathematics at Case Western Reserve University.

Prof. Munch’s road to mathematics is interesting in that music plays a significant role in her life. In addition to her mathematical degrees, she also holds a degree in Harp performance from the Eastman School of Music. Find out how her musical studies affected her discovery of mathematics. In her interview, she mentions Takens embeddings, and the cover features a kind of Takens embedding. The embedding is actually a 4-dimensional Takens embedding consisting of two spatial dimensions and two that extend into the color space.

Statistics plays an important role in so many sciences. Research would grind to a halt without it. One of the most important results in statistics is the central limit theorem. Prof. Meckes has contributed an eloquent two-part article that explains the meaning of this theorem. In part 1, she explains the laws of probability.

In Math In Your World, Lightning Factorial uses statistics to improve at darts, following the lead of statisticians Ryan Tibshirani, Andrew Price, and Jonathan Taylor, who showed in their paper *A Statistician Plays Darts*, that depending on your dart throwing prowess, you might be better off not aiming for the bull’s-eye. Skeptical? Read Lightning’s article!

In Anna’s Math Journal, Anna continues her investigation of irreducible polynomials over the finite field with 2 elements. This investigation traces back to suggested exercises made by Prof. Judy Walker in the previous issue. If you’re unfamiliar with finite fields but want to follow along with Anna on her journey, this issue’s Learn by Doing is just for you. In it, finite fields are introduced assuming very little by way of prerequisites.

Meanwhile, Emily and Jasmine continue their quest for “nice” triangles. This time, they apply a technique that is often used to find Pythagorean triples to find formulas that yield the sides of all primitive triangles that contain a 60 degree angle, such as the 5-7-8 and 16-19-21 triangles.

We conclude with some notes from the club! We’ve got a wonderful group of members this semester and if you’re a girl in grades 5-12 who lives near Cambridge, MA, you’re welcome to attend!

We hope you enjoy it!

Finally, a reminder: when you subscribe to the Girls’ Angle Bulletin, you’re not just getting a subscription to a magazine. You are also gaining access to the Girls’ Angle mentors. We urge all subscribers and members to write us with your math questions or anything else in the Bulletin or having to do with mathematics in general. We will respond. We want you to get active and *do* mathematics. Parts of the Bulletin are written to induce you to wonder and respond with more questions. Don’t let those questions fade away and become forgotten. Send them to us!

We continue to encourage people to subscribe to our print version, so we have removed some content from the electronic version. Subscriptions are a great way to support Girls’ Angle while getting something concrete back in return. We hope you subscribe!

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(Added October 1, 2015: This raffle is now closed. Thank you to all who entered. Congratulations to Fran M. who won the general draw and Iris L. who won the member draw!)

Can you reconstruct Serena and Katherine’s path through the museum?

They start at the museum entrance. They choose a random direction, north, south, east, or west and walk together in that direction until they come upon the first exhibit that they had not yet visited and stop. When done looking at that exhibit, they again choose a random direction, north, south, east, or west and walk together in that direction until they come upon an exhibit that they had not yet visited and stop. They continue wandering in this manner. Miraculously, they manage to visit all 17 exhibits!

Figure out all 17 directions Serena and Katherine took and the order that they took them. Send your answer to girlsanglepuzzler “at” gmail.com by **midnight on September 30, 2015**. For example, you might send the string “NSEWNSEWNSEWNSEWN” if you think those are the directions they went in that order, left to right. On October 1, 2015, two random “winners” will be drawn from correct entries, one from among Girls’ Angle members and the other from among the general public. The two “winners” will receive a modest prize. (The member prize is bigger.)

Exhibits are marked by flag poles. Determine the path that goes from flag pole to flag pole. They start at the museum entrance and end at one of the exhibits.

Good luck!

(We will not use your contact information for any purpose other than to deliver your prize, should you win. After the winner has been selected, all emails received will be promptly deleted. At the winner’s discretion, we will let you know who won. Anyone who makes more than one submission will be disqualified! Sorry! Also, this offer is only valid in those states in the United States where such things are legal. There is no fee to enter this puzzle contest.)

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The object on the cover of Volume 8, Number 6 is called the Chebyshev Lollipop. It is based directly on an idea of Michael Trott. The mathematical content differs slightly from his creation which can be seen at MathWorld.

Chebyshev polynomials (of the first kind) appear in this summer’s batch of Summer Fun problem sets and in a new Emily and Jasmine series which commences in this issue.

In school, Jasmine happened by a geometry class where the teacher had the peculiar figure shown at right on the board. That figure turned out to be the launch point for an adventure in search of nice triangles.

This issue’s interview is with the Chair of the Department of Mathematics at the University of Nebraska-Lincoln, Professor Judy Walker. In her interview, Prof. Walker gives some pointers for how to learn mathematics well, saying “I absolutely must work through examples.”

In Anna’s Math Journal, Anna takes up Prof. Walker’s specific example suggestion and explores finite fields with 4 and 8 elements

In Part 5 of our series on the derivative, we explore the derivative of the exponential function.

We wrap up with detailed solutions to this summer’s batch of Summer Fun problem sets which include proofs of the arithmetic-geometric mean inequality, a derivation of the Taylor series of the arctangent function, and a proof of Lagrange’s theorem that, in finite groups, the order of any subgroup divides the order of the group.

We hope you enjoy it!

Finally, a reminder: when you subscribe to the Girls’ Angle Bulletin, you’re not just getting a subscription to a magazine. You are also gaining access to the Girls’ Angle mentors. We urge all subscribers and members to write us with your math questions or anything else in the Bulletin or having to do with mathematics in general. We will respond. We want you to get active and *do* mathematics. Parts of the Bulletin are written to induce you to wonder and respond with more questions. Don’t let those questions fade away and become forgotten. Send them to us!

We continue to encourage people to subscribe to our print version, so we have removed some content from the electronic version. Subscriptions are a great way to support Girls’ Angle while getting something concrete back in return. We hope you subscribe!

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I haven’t seen this proof before, but there are so many proofs of the Pythagorean theorem that it wouldn’t surprise me if this has already been discovered. If you’ve seen this proof published elsewhere, please send email to Girls’ Angle and let me know.

I am aware of other “origami” proofs of the Pythagorean theorem, although the ones I’ve seen don’t particularly use origami so much as that they take some diagram used to illustrate a proof and then render the lines in the diagram as creases in paper.

This particular proof is not elegant, but it does show how one might discover the Pythagorean theorem through a natural line of exploration starting with an origami square. Also, it really uses the notion of an origami fold as its key ingredient.

All you have to do is think about what happens when you fold a corner of the origami square to a point along one of the further edges, as illustrated below.

That single fold produces 3 triangles and a quadrilateral.

Notice that the 3 triangles are all similar right triangles. (Here, we must use the fact that a square has 4 right angles.)

Since the entire model is determined by the parameter , a natural question to ask is: What are the lengths of all the other line segments in terms of ?

To get the answer, we will repeatedly apply two facts. First, that the 3 triangles are similar to each other. And second, that the edge lengths along a side of the square add up to the side length of the square, which we’ll go ahead and take to be one unit for now.

We’ll start by letting be the length of the other leg of . We’ll then express the other lengths in terms of and and find an equation that relates the two. By solving for in terms of , we will effectively have succeeded in expressing the lengths in terms of .

Here goes!

We add the label to the diagram:

The hypotenuse of and the leg of length make up what was the right side of our square, so the hypotenuse of has length :

One of the legs of extends to the full side of the square:

Using the fact that and are similar, we can find the other side lengths of :

One leg of and the hypotenuse of form what was the upper side of the origami square:

Next, we use similarity of with to see that its other sides have length:

and .

So far, we’ve used the right, bottom, and top sides of the original square. So now we use the left side. Studying the figure, we see that the original left side of the square has become one leg of and the last two sides we computed of . Thus,

.

If you solve for in terms of using this equation, you will find that .

Thus, the legs of are and and its hypotenuse has length .

Notice that as varies from 0 to 1, the ratio of the leg lengths of covers the entire range from 0 to infinity (i.e. the range of is when is restricted to the interval ). This means that a representative of every similarity class of right triangle can be folded by making a suitable choice of .

Therefore, by scaling the origami square if necessary, we can make congruent to any given right triangle.

The form of the expressions for the lengths of the sides of in terms of suggests the possibility of finding an explicit relationship between these 3 side lengths, especially since one of the side lengths is just , and if we can find such a relationship that is also homogeneous in these lengths, then the relationship will hold for *all* right triangles since a homogeneous relationship is preserved by scaling. And, indeed, the desired algebraic identity is , hence, the Pythagorean theorem. (Making note of this algebraic identity without knowing the Pythagorean theorem might be a bit of a trick, but it doesn’t seem unreasonable to think it possible.)

(Note that as well, and this corresponds to the equation , where and are the legs of a right triangle with hypotenuse of length . However, this identity is *not* homogeneous in the leg lengths, so it is not true that for all right triangles. The equation holds only for right triangles where , i.e. those folded from a unit square.)

In an earlier post, we described David Gale’s method for constructing Pythagorean triples. There, our aim was to find Pythagorean triples, not prove the Pythagorean theorem, so when we spoke of finding the lengths of , we had in mind a method that made use of the Pythagorean theorem. One could say that the content of this blog post is that you don’t have to assume the Pythagorean theorem there because you can deduce it instead. In other words, one-fold origami can deliver the Pythagorean theorem from scratch as well as all Pythagorean triples.

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Volume 8, Number 5 of the Bulletin kicks off with an interview of Ivana Alexandrova. Ivana is an Assistant Professor of Mathematics at the State University of New York, Albany. Among other things, she maintains a webpage of weekly problems for high school students. Check it out!

The topic of induction came up quite a few times this spring at the Girls’ Angle club, so next comes an article on this widely used proof technique.

This issue’s Learn by Doing features irrational numbers and culminates in a series of problems that let you reconstruct a proof of the irrationality of due to Charles Hermite.

Anna tackles one of Prof. Alexandrova’s weekly problems for high school students in Anna’s Math Journal, finding 3 different ways to solve the problem, which is to compute . Can you find your own solution?

Next comes our 4th installment on the derivative where we find the derivatives of the basic trigonometric functions. The way we deduce the derivative of sine is similar in spirit to the way we showed that the area under one hump of a sine curve is exactly 2.

Since this is our June issue, we include the 2015 Summer Fun problem sets. This batch contains problems pertaining to telescoping series (by Fan Wei), induction, the symmetric group (by Noah Fechtor-Pradines), and derivatives.

We hope you enjoy it!

*do* mathematics. Parts of the Bulletin are written to induce you to wonder and respond with more questions. Don’t let those questions fade away and become forgotten. Send them to us!

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First, the problem:

Let be a diameter of circle . Extend through to . Point lies on so that line is tangent to . Point is the foot of the perpendicular from to line . Suppose , and let denote the maximum possible length of the segment . Find .

Here’s an illustration of the situation:

We must maximize the length of the red line segment .

As the diameter rotates, both and move. It’s typically easier to maximize the length of a line segment if one of its endpoints is stationary, so let’s instead maximize the length of the mid-segment of triangle , specifically, the mid-segment that joins the center of and the midpoint of (which are labeled and , respectively, in the following figure):

As diameter rotates, what path does trace? Since is the midpoint of , the point traces out the curve obtained by squashing the circle by a factor of 1/2 in the vertical direction:

reaches its maximum length precisely when is perpendicular to the tangent to the squashed circle at . The slope of the tangent to the squashed circle at is half the slope of the tangent to at point . Therefore, we want the slope of to be twice the slope of (since *is* perpendicular to the tangent to at ), which is the same thing as saying that we want to be twice .

Since is the midpoint of , we see that is maximized exactly when , i.e. when and trisect the segment .

Thus, is maximized when and . Hence, the maximal value of is .

Remembering that is half that of , we conclude that the answer to the AIME problem is .

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Volume 8, Number 4 of the Bulletin kicks off with the concluding half of our 2-part interview with Hamilton College Assistant Professor of Mathematics Courtney Gibbons. In this second half, one of the objects she describes are the Cayley graphs of groups. This inspired the creation of several Cayley graphs by members of Girls’ Angle, which are featured in this issue’s Math Buffet.

The cover itself also shows a Cayley graph of .

Next, Emily and Jasmine continue their exploration of *n*-pointed stars. This time, they follow-up on an observation that Emily made about the (17, 7)-star on the cover of Volume 8, Number 3. She noticed that the (17, 7)-star contains (17, *k*)-stars for *k* = 1, 2, 3, 4, 5, and 6. Together, they not only show that this is an instance of a general phenomenon, but they also find the relationship between the sizes of such embedded stars within stars.

Anna wraps up her study of cross sections of the surface *z* = *xy*.

In Part 3 of The Derivative, we explain both the Chain Rule and the Quotient Rule along the line emphasized in this series: local linearity. We hope that the Chain Rule, especially, appears “obvious” from this point-of-view. For another take on the Chain Rule, here’s an earlier post that uses movies to explain it.

Next, Stuart Sidney, Emeritus Professor at the University of Connecticut, entertains us with curious facts about palindromic numbers.

We introduce a mathematical variant of the classic Hot Potato game that can be used as a vehicle to explore quite a few math concepts and in quite a bit of depth.

Finally, in Notes From the Club, we give a brief account of Bathsheba Grossman’s recent Support Network visit to Girls’ Angle. If you haven’t seen her mathematically inspired sculptures, you’re in for a treat. She mesmerized us for over an hour with her inspiring creations.

We hope you enjoy it!

*do* mathematics. Parts of the Bulletin are written to induce you to wonder and respond with more questions. Don’t let those questions fade away and become forgotten. Send them to us!

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We claim that every point in space on the side of the plane with the rolling snowball and within a snowball diameter of the plane can be visited by the marked point.

Let *C* be the circumference of the snowball. Any positive distance less than 2*C* can serve as the base of an isosceles triangle with equal sides of length *C*. Draw this triangle in the plane and put the snowball at one endpoint of the base with its marked point touching the plane. Roll the snowball along the two sides of length *C*. It ends up at the other endpoint of the base with its marked point touching the plane. This shows that the marked point can visit any point within a circle of radius 2*C* from its starting point. By taking several such journeys, we see that every point in the plane can be visited. For any point *P* on the same side of the plane as the snowball and within a snowball’s diameter of the plane, position the snowball so that its marked point is at *P*. We can roll this snowball until the marked point is touching the plane and since all points in the plane are visitable, we conclude that *P* can also be visited.

We claim that the locus of visitable points is a cardioid of revolution.

Let’s mark the point on the sphere where the marked point on the snowball touches it at the start with a golden dot. Look at the situation from the point-of-view of the common tangent plane. The two balls roll as mirror images of each other. The marked points are reflections of each other in the common tangent plane.

In other words, no matter how the snowball rolls about the sphere, the marked point will be located at the reflection of the golden dot in the common tangent.

We conclude that the locus of visitable points is a surface with rotational symmetry (about the axis one of whose poles is the golden dot). A vertical cross section by a plane containing the pole of the stationary sphere is the curve traced out by a dot on a circle as it rolls about a circle of the same diameter, and this is known to be a cardioid.

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