sandra123 wrote:

If x and y are numbers such that (x+9)(y-9)=0, what is the smallest possible value of x^2 + y^2

A. 0

B. 9

C. 18

D. 81

E. 162

I understand its a basic question, but I fail to understand the concept. I simply put x+9=0, so x=-9. in the same way, y-9=0, so y=9

so x^2 + y^2 = 81+81=162

what am I doing wrong?

This question could me solved in this way.

(x+9)(x-9) = 0

=> xy - 81 = 9(x-y)

=> (x-y) = (xy/9) - 9

=> Squaring on both sides and cancelling equal terms

=> x^2 + y^2 = (xy)^2/81 + 81

Notice that 81 is added to (xy)^2/81, so the absolute minimum value of the above expression will be equal to 81 when x=9 and y=9 => x^2 + y^2 = 162.