# 1975 AHSME Problems/Problem 5

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## Problem

The polynomial $(x+y)^9$ is expanded in decreasing powers of $x$. The second and third terms have equal values when evaluated at $x=p$ and $y=q$, where $p$ and $q$ are positive numbers whose sum is one. What is the value of $p$? $\textbf{(A)}\ 1/5 \qquad \textbf{(B)}\ 4/5 \qquad \textbf{(C)}\ 1/4 \qquad \textbf{(D)}\ 3/4 \qquad \textbf{(E)}\ 8/9$

## Solution

Solution by e_power_pi_times_i

The second and third term of $(x+y)^9$ is $9x^8y$ and $36x^7y^2$, respectively. For them to be equal when $x = p$, $\dfrac{p}{4} = y$. For them to be equal when $y = q$, $x = 4q$. Then $p+\dfrac{p}{4} = q+4q$, so $\dfrac{5p}{4} = 5q$, which simplifies to $p = 4q$. Since $p+q = 1$, $p = \boxed{\textbf{(B) } 4/5}$.

## See Also

 1975 AHSME (Problems • Answer Key • Resources) Preceded byProblem 4 Followed byProblem 6 1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 • 26 • 27 • 28 • 29 • 30 All AHSME Problems and Solutions

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