# 2010 Solutions. a + b. a + b 1. (a + b)2 + (b a) 2. (b2 + a 2 ) 2 (a 2 b 2 ) 2

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1 00 Problem If a and b are nonzero real numbers such that a b, compute the value of the expression ( ) ( b a + a a + b b b a + b a ) ( + ) a b b a + b a +. b a a b Answer: 8. Solution: Let s simplify the monstrous expression one step at a time: ( ) ( b a + a a + b b b a + b a ) ( + ) a b b a a + b b a = b4 + a 4 a b (a + b) + (b a) a b (b a)(a + b) ( b + a a + b = (b a ) (a + b ) (b + a ) (a b ) a b b a (a b )(b + a ) = (b a )(a + b ) 4a b a b (a b )(b + a ) = 8(b a ) a b = 8. a b a b b + a ) Problem Jane has two bags X and Y. Bag X contains 4 red marbles and 5 blue marbles (and nothing else). Bag Y contains 7 red marbles and 6 blue marbles (and nothing else). Jane will choose one of her bags at random (each bag being equally likely). From her chosen bag, she will then select one of the marbles at random (each marble in that bag being equally likely). What is the probability that she will select a red marble? Express your answer as a fraction in simplest form. Answer: 5 34.

2 AT Math Prize for Girls 00 Solution: The probability that Jane will choose bag X and then select a red marble is Pr(X and red) = 4 9 = 9. The probability that Jane will choose bag Y and then select a red marble is Pr(Y and red) = 7 3 = 7 6. So the probability that Jane will select a red marble is the sum Pr(red) = Pr(X and red) + Pr(Y and red) = (6) + 9(7) = 9(6) = 34 = Problem 3 How many ordered triples of integers (x, y, z) are there such that x + y + z = 34? Answer: 48. Solution: We have to represent 34 as the sum of three squares chosen from 0,, 4, 9, 6, and 5. By playing a bit, we see that the only such sums (ignoring order) are and Let s consider the two cases separately. Case : Since x, y, and z are 5, 9, and 0 (in some order), x, y, and z are ±5, ±3, and 0 (in some order). There are 3! = 6 ways to choose the order, and = 4 ways to choose the ± signs. So the number of such triples is 6 4, or 4. Case : Since x, y, and z are 6, 9, and 9 (in some order), x, y, and z are ±4, ±3, and ±3 (in some order). There are 3 places for ±4,

3 AT Math Prize for Girls 00 and 3 = 8 ways to choose the ± signs. So the number of such triples is 3 8, or 4. There are 4 triples in the first case and 4 triples in the second case, so the total number of triples is 48. Problem 4 Consider the sequence of six real numbers 60, 0, 00, 50, 30, and x. The average (arithmetic mean) of this sequence is equal to the median of the sequence. What is the sum of all the possible values of x? (The median of a sequence of six real numbers is the average of the two middle numbers after all the numbers have been arranged in increasing order.) Answer: 35. Solution: Excluding x, the sum of the five numbers is 350. So the sum of all six numbers is x Hence their mean is x Excluding x, the five numbers in order are 0, 30, 60, 00, and 50. The median of these five numbers is 60. Including x, the median will depend on the value of x. Case x 30: In this case, the median is = 45. So we get the equation x+350 = 45. The solution is x = 80. Since 80 is less than 30, it 6 is a possible value of x. Case 30 < x < 00: In this case, the median is x+60. So we get the equation x+350 = x+60. The solution is x = 85. Since 85 is between 30 and 6 00, it is another possible value of x. Case x > 00: In this case, the median is = 80. So we get the equation x+350 = 80. The solution is x = 30. Since 30 is bigger than 00, 6 it is a third possible value of x. Combining all three cases, we see that the three possible values of x are 80, 85, and 30. Their sum is 35. Problem 5 Find the smallest two-digit positive integer that is a divisor of Answer:. Solution: Let N = We re looking for a small two-digit divisor of N, so let s start from 0 and work up. Is 0 a divisor of N? No, because N doesn t end with a 0. 3

4 AT Math Prize for Girls 00 Is a divisor of N? Let s use the divisibility rule for, which involves the alternating sum of the digits of N: = 9 3 = 6. Because 6 is not divisible by, neither is N. Is a divisor of N? We have to check whether N is divisible by 3 and 4. It is divisible by 4, because its last two digits are, which is divisible by 4. To check whether N is divisible by 3, let s compute the sum of the digits of N: =. Because this sum is divisible by 3, so is N. Since N is divisible by both 3 and 4, it is divisible by. Hence the smallest two-digit divisor of N is. Problem 6 The bases of a trapezoid have lengths 0 and, and the legs have lengths 34 and 3 5. What is the area of the trapezoid? Express your answer as a fraction in simplest form. Answer: 93. Solution: Below is a drawing of the trapezoid and two of its altitudes x 0 x By the Pythagorean theorem applied to the left triangle, the height of the trapezoid is 34 x. By Pythagoras applied to the right triangle, the height is 45 ( x). So we get the equation 34 x = 45 ( x) = 45 ( x + x ) = 76 + x x. The quadratic terms cancel. Solving for x gives x = = 0 = 5. 4

5 AT Math Prize for Girls 00 As a result, the height of the trapezoid is 34 x = 34 5 = 34 5 = 9 = 3. So the area of the trapezoid is = 3 3 = 93. Problem 7 The graph of (x + y ) 3 shown in the figure below. y = x y 3 is a heart-shaped curve, x For how many ordered pairs of integers (x, y) is the point (x, y) inside or on this curve? Answer: 7. Solution: The equation of the heart is (x + y ) 3 = x y 3. Plugging in y = 0, the equation becomes x = 0, so x = ±. Plugging in y =, the equation becomes (x ) 3 = x, so x is, 0, or. Plugging in y =, the equation becomes (x ) 3 = x, so x = 0. We have identified 6 integer points on the heart, as shown in the picture below. 5

6 AT Math Prize for Girls 00 y x The points inside the heart satisfy (x + y ) 3 < x y 3. So the origin (0, 0) is inside the heart. Looking at the picture, we see that every point in or on the heart satisfies y <. So we have found all the integer points in or on the heart. Hence the number of such points is 6 + = 7. Problem 8 When Meena turned 6 years old, her parents gave her a cake with n candles, where n has exactly 6 different positive integer divisors. What is the smallest possible value of n? Answer: 0. Solution: To have 6 positive divisors, n must be of the form p 5, p 7 q, p 3 q 3, p 3 qr, or pqrs, where p, q, r, and s are different primes. Let s find the smallest n in each of these five cases. Case n = p 5 : In this case, the smallest n is 5, or 3,768. Case n = p 7 q: In this case, the smallest n is 7 3, or 384. Case n = p 3 q 3 : In this case, the smallest n is 3 3 3, or 6. Case n = p 3 qr: In this case, the smallest n is 3 3 5, or 0. Case n = pqrs: In this case, the smallest n is 3 5 7, or 0. Looking at all five cases, we see that the smallest possible value of n is 0. Problem 9 Lynnelle took 0 tests in her math class at Stanford. Her score on each test was an integer from 0 through 00. She noticed that, for every 6

7 AT Math Prize for Girls 00 four consecutive tests, her average score on those four tests was at most What is the largest possible average score she could have on all 0 tests? Answer: 57. Solution: Let the 0 scores be s, s,..., s 0. For each four consecutive scores, their average is at most 47.5, so their sum is at most , which is 90. In particular, we have Adding all three inequalities, we get s + s + s 3 + s 4 90 s 4 + s 5 + s 6 + s 7 90 s 7 + s 8 + s 9 + s s + s + s 3 + s 4 + s 5 + s 6 + s 7 + s 8 + s 9 + s So the sum of all 0 scores is at most 570. Hence the average of all 0 scores is at most 57. Let s show that an average score of 57 is possible. Consider the 0 scores 95, 95, 0, 0, 95, 95, 0, 0, 95, 95. Each score is an integer from 0 through 00. For each four consecutive scores, their sum is 90, and so their average is The sum of all 0 scores is 570, and so their average is 57. Hence an average score of 57 is possible. Combining our observations, we see that the largest possible average score is 57. Problem 0 The triangle ABC lies on the coordinate plane. The midpoint of AB has coordinates ( 6, 63), the midpoint of AC has coordinates (3, 50), and the midpoint of BC has coordinates (6, 85). What are the coordinates of point A? Express your answer as an ordered pair (x, y). Answer: ( 9, 7). Solution: Because the midpoint of AB is ( 6, 63), we get the equation A + B = ( 6, 63). 7

8 AT Math Prize for Girls 00 Similarly, we get the equations A + C = (3, 50) and B + C = (6, 85). Adding the first two equations gives A + B + C = ( 3, 3). Substituting the third equation into the last equation gives Solving for A gives A + (6, 85) = ( 3, 3). A = ( 6, 85) + ( 3, 3) = ( 9, 7). Problem In the figure below, each side of the rhombus has length 5 centimeters. 60 The circle lies entirely within the rhombus. The area of the circle is n square centimeters, where n is a positive integer. Compute the number of possible values of n. Answer: 4. Solution: Let s consider the case when the circle and rhombus have the same center and are tangent to each other. See the following picture. 8

9 AT Math Prize for Girls 00 B E O 30 A Because triangle ABO is a triangle with AB = 5 cm, we have OA = 5 3 cm. Because triangle AEO is a triangle, we have OE = 5 3 cm. In other words, the radius of the circle (in centimeters) is r = Thus the area of the circle (in square centimeters) is ( ) 5 A = πr = π 3 = π 5 75π (3) = Because π < 3., the area satisfies the upper bound A = 75π 6 < 75(3.) 6 = 75(0.) = 5. Because π > 3, the area satisfies the lower bound A = 75π 6 > 75(3) = > 4. So the area of this big circle is between 4 and 5. It s clear that no circle inside the rhombus can be larger than the circle above. By shrinking our big circle, we can have circles of area,,..., 4. So the number of possible integer areas is 4. Problem Say that an ordered triple (a, b, c) is pleasing if. a, b, and c are in the set {,,..., 7}, and. both b a and c b are greater than 3, and at least one of them is equal to 4. How many pleasing triples are there? Answer: 8. 9

10 AT Math Prize for Girls 00 Solution: The value of b can be 5, 6,..., 3. Let s fix one of these 9 values of b. The possible values of a split into two cases. Case a b 5: Then c must be b + 4. Since there are b 5 possible values of a, there are b 5 pleasing triples in this case. Case a = b 4: Then c could be b + 4, b + 5,..., 7. There are 4 b possible values of c, so there are 4 b pleasing triples in this case. Combining both cases, we see that for each fixed b, the number of pleasing triples is (b 5) + (4 b) = 9. Since there are 9 possible values of b, the total number of pleasing triples is 9 9, or 8. Problem 3 For every positive integer n, define S n to be the sum 00 ( S n = cos k! π ) n. 00 k= As n approaches infinity, what value does S n approach? Answer: 944. Solution: The prime factorization of 00 is So let s split the sum into two cases, depending on how the index k compares with 67. Case k 66: In this case, k! is not a multiple of 67, and so is not a multiple of 00. Hence k! π k! π is not a multiple of π. Thus cos is strictly between and. Therefore as n approaches infinity, (cos k! π 00 )n approaches zero. Case k 67: In this case, k is a multiple of, 3, 5, and 67, and so is a multiple of 00. Hence k! π k! π is a multiple of π. Thus cos is Therefore (cos k! π 00 )n is always. Combining the two cases, we see that each k 66 contributes nothing to the limit, while each k 67 contributes. The number of k from 67 to 00 is = = 944. So the sum approaches 944. Problem 4 In the figure below, the three small circles are congruent and tangent to each other. The large circle is tangent to the three small circles. 0

11 AT Math Prize for Girls 00 The area of the large circle is. What is the area of the shaded region? Express your answer in the form a n b, where a and b are positive integers and n is a square-free positive integer. Answer: Solution: Let R be the radius of the large circle. Because the area of the large circle is, we have πr =. Let r be the radius of the three small circles. Consider the equilateral triangle whose vertices are the centers of the three small circles, as shown below. r r r r r r r Each side of the equilateral triangle has length r. By properties of triangles, each median of this triangle has length r 3. Because the centroid of the triangle divides each median in a : ratio, we have R r = 3 r 3. Clearing fractions gives 3R 3r = r 3. Solving for r gives r = So the area of each small circle is R = ( 3 3)R. πr = π( 3 3) R = ( 3 3) = = 3.

12 AT Math Prize for Girls 00 The combined area of all three small circles is thus 3( 3) = Hence the area of the shaded region is ( ) = Problem 5 Compute the value of the sum + tan tan tan tan tan tan tan tan tan Answer: 5. Solution: The first term is + tan 3 0 = = + 0 =. Because 0 and 80 are complements, tan 0 and tan 80 are reciprocals. So the second and ninth terms add up to + tan tan 3 80 = + tan tan 3 0 ( + tan 3 0 )( + tan 3 80 ) + tan tan 3 80 = + tan tan tan 3 0 tan 3 80 Similarly, we have = + tan3 0 + tan tan tan =. + tan tan 3 70 = + tan tan 3 60 = + tan tan 3 50 =. Adding all five displayed equations, we see that the entire sum is 5.

13 AT Math Prize for Girls 00 Problem 6 Let P be the quadratic function such that P (0) = 7, P () = 0, and P () = 5. If a, b, and c are integers such that every positive number x less than satisfies P (n)x n = ax + bx + c ( x) 3, n=0 compute the ordered triple (a, b, c). Answer: (6,, 7). Solution: From the given initial conditions, we have P (n)x n = 7 + 0x + 5x n=0 Multiplying by x gives ( x) P (n)x n = x n=0 Multiplying by x again gives ( x) P (n)x n = 7 4x + x n=0 Multiplying by x a third time gives ( x) 3 P (n)x n = 7 x + 6x n=0 Comparing coefficients, we see that a = 6, b =, and c = 7. So the desired triple is (6,, 7). Problem 7 For every x, there is a unique number W (x) such e that W (x)e W (x) = x. The function W is called Lambert s W function. Let y be the unique positive number such that y log y =

14 AT Math Prize for Girls 00 The value of y is of the form e W (z ln ) for some rational number z. What is the value of z? Express your answer as a fraction in simplest form. Answer: 5 3. Solution: Replacing y with the expression involving z, we have 3 5 = = y log y (z ln ) e W log e W (z ln ) W (z ln ) e = W (z ln ) log e = W (z ln )e W (z ln ) log e = z ln() log (e) = z. Solving for z, we get z = 5 3. Problem 8 If a and b are positive integers such that = a cos π b, compute the ordered pair (a, b). Answer: (4, 4). Solution: Let s first simplify 768: 768 = 56 3 = 6 3. Since the problem asks for cosine, let s express 768 with cosine: 768 = 6 3 = 3 3 = 3 cos π 6. 4

15 AT Math Prize for Girls 00 Next, we have = cos π6 = 4 + cos π 6. How can we simplify a square root of a cosine? We can use the half-angle identity cos x + cos x =, where x is any number between π and π. So we get = 4 + cos π6 + cos π = 8 6 = 8 cos π. Now let s use the half-angle identity one more time: = cos π + cos π = 4 = 4 cos π 4. So a = 4 and b = 4 works. By case analysis on a, we see that no other pair works. Hence the answer is (4, 4). Problem 9 Let S be the set of 8 points (x, y) such that x and y are integers from 4 through 4. Let A, B, and C be random points chosen independently from S, with each of the 8 points being equally likely. (The points A, B, and C do not have to be different.) Let K be the area of the (possibly degenerate) triangle ABC. What is the expected value (average value) of K? Express your answer as a fraction in simplest form. Answer: Solution: Let A = (x, y ), let B = (x, y ), and let C = (x 3, y 3 ). By the shoelace formula (also called the surveyor s area formula), the area of ABC is K = x y + x y 3 + x 3 y x y x 3 y x y 3. Squaring gives K = 4 ( x y + x y 3 + x 3y + x y + x 3y + x y 3 + cross terms ). 5

16 AT Math Prize for Girls 00 The cross terms are terms like x x y y 3 or x y y 3 ; in each cross term, at least one variable is not squared. Writing E for expected value, we shall take the expected value of both sides of the last equation: E(K ) = [ E(x 4 y) + E(x y3) + E(x 3y) + E(x y) + E(x 3y) + E(x y3) ] + E(cross terms). Let s analyze the first term E(x y ). Because x and y are independent, we have E(x y ) = E(x )E(y ). We can compute E(x ) as follows: E(x ) = ( 4) + ( 3) = ( ) 9 = (30) 9 = 0 3. Similarly, E(y) is 0. So 3 E(x y) is ( 0 3 each have this value. Hence E(K ) = 4 ), which is 400. The first 6 terms 9 [ ] + E(cross terms) 9 = E(cross terms). 4 Let s analyze one of the cross terms like x y y 3. By independence, we have E(x y y 3 ) = E(x )E(y )E(y 3 ). By symmetry, E(y ) is zero, and so the whole expression is zero. In the same way, the expected value of every cross term is zero. We can thus ignore the cross terms. The expected value of K is

17 AT Math Prize for Girls 00 Problem 0 What is the value of the sum z z, where z ranges over all 7 solutions (real and nonreal) of the equation z 7 =? Express your answer as a fraction in simplest form. Answer: Solution: Let z satisfy z 7 =. By the formula for geometric series, we have + z + z + z 3 + z 4 + z 5 + z 6 = z7 z = ( ) z Dividing by, we have z = 6 z j. j=0 Taking the squared magnitude of both sides, we get Because ω = ωω, we have z = 4 6 z j. j=0 = z. z = 4 6 j=0 z j 6 z k = 4 k=0 6 j=0 z j 6 z k = 4 k=0 6 6 z j k. j=0 k=0 Now let s sum over all such z. We get z z = 4 6 j=0 k=0 6 z j k. z If j = k, then we have z j k = z z z 0 = z = 7. 7

18 AT Math Prize for Girls 00 Otherwise, j k is a nonzero integer between 6 and 6. Because the values of z form a geometric sequence whose common ratio is a 7th root of unity (different from ), so do the values of z j k ; in particular, their sum is z j k = 0. Hence the total sum is z z z = 4 6 j=0 7 = =

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