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2020 AMC 10A Problem 14

Problem 14 of 25IntermediateAlgebra

Real numbers xx and yy satisfy x+y=4x + y = 4 and xy=2.x \cdot y = -2. What is the value of x+x3y2+y3x2+y?x + \frac{x^3}{y^2} + \frac{y^3}{x^2} + y?

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Solution

Let Sk=xk+ykS_k=x^k+y^k. Since x+y=4x+y=4 and xy=2xy=-2, the numbers xx and yy satisfy t24t2=0t^2-4t-2=0, so Sk=4Sk1+2Sk2S_k=4S_{k-1}+2S_{k-2}. Using S0=2S_0=2 and S1=4S_1=4, we get S2=20S_2=20, S3=88S_3=88, S4=392S_4=392, and S5=1744S_5=1744. The expression is x+y+x5+y5x2y2x+y+\dfrac{x^5+y^5}{x^2y^2} =4+17444=440=4+\dfrac{1744}{4}=440. Thus, D is the correct answer.

More practice

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