Math question help?

Question

If xy= 8 and x² + y² = 20, what is one possible value of x + y?

Answer 1

(x+y)²= x²+2xy+y².   This is a clever trick for these problems. Note that we can computer the values on the right with the given information.

 

(x+y)²= 20+2(8) = 36

 

So x+y could be 6 or -6