Question

If x^{2} + y^{2} = 6" align="absmiddle" class="latex-formula"> and $x + y = \sqrt{7}$, then what is $x-y$?

Answer 1

Recall that

(x + y)² = x² + 2xy + y²

Then if x + y = √7 and x² + y² = 6, we have

(√7)² = 6 + 2xy   ⇒   2xy = 1

We also have

(x - y)² = x² - 2xy + y²

so that

(x - y)² = 6 - 1   ⇒   (x - y)² = 5

which means x - y has two possible values, √5 or -√5.