Question

If x^2+25y^2=416 and x-5y=-16 the value of xy is

a)16
b)8
c)-8
d)-16

Answer 1

(a)

Explanation:

Note the expansion of (x - 5y)² is

(x - 5y)² = x² - 10xy + 25y² , that is

(x - 5y)² = x² + 25y² - 10xy ← substitute given values

(- 16)² = 416 - 10xy

256 = 416 - 10xy ( subtract 416 from both sides )

- 160 = - 10xy ( divide both sides by - 10 )

xy = 16 → (a)