Real numbers x and y satisfy x+y=4 and x⋅y=−2. What is the value of x+y2x3+x2y3+y?
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Solution
Let Sk=xk+yk. Since x+y=4 and xy=−2, the numbers x and y satisfy t2−4t−2=0, so Sk=4Sk−1+2Sk−2.
Using S0=2 and S1=4, we get S2=20, S3=88, S4=392, and S5=1744. The expression is x+y+x2y2x5+y5=4+41744=440. Thus, D is the correct answer.