SOLUTION: If x-y=4 and xy=21 find x^3-y^3

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Question 1037399: If x-y=4 and xy=21 find x^3-y^3
Answer by ikleyn(52781) About Me  (Show Source):
You can put this solution on YOUR website!
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If x-y=4 and xy=21 find x^3-y^3
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x%5E3-y%5E3 = %28x-y%29%2A%28x%5E2+%2B+xy+%2B+y%5E2%29   (which is classic identity)

= %28x-y%29%2A%28%28x-y%29%5E2+%2B+3xy%29   (now plug-in the given data)

= 4*(4^2 +3*21) = 4*(16+63) = 4*79 = 316.