Question

If x²+y²=20 and x-y=6 how is x.y?
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Answer 1

`(x-y)^2=x^2-2xy+y^2`

Therefore

`6^2=20-2xy\\36=20-2xy\\2xy=-16\\xy=-8`

Answer 2

xy = - 8

Explanation:

note that

(x - y)² = x² + y² - 2xy , that is

6² = 20 - 2xy

36 = 20 - 2xy ( subtract 20 from both sides )

16 = - 2xy ( divide both sides by - 2 )

- 8 = xy