SOLUTION: If x + y = 7 and xy = 5, what is x^3 + y^3?

Question 1017466: If x + y = 7 and xy = 5, what is x^3 + y^3?
Answer by ikleyn(52754) (Show Source): You can put this solution on YOUR website!
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If x + y = 7 and xy = 5, what is x^3 + y^3?
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First let us make these equivalent transformations:

 =  =  + .

So you have this identity:

 =  + .

Now let us substitute the given values x + y = 7 and xy =5 into this identity. You will get

 =  + 3*5*7.

Hence, 

 =  - 3*5*7 = 343 - 105 = 238.

Closely related to this problem is the lesson HOW TO evaluate expressions involving x + 1/x, x^2 + 1/x^2 and x^3 + 1/x^3 in this site.

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