Question

Find (fg)(-4) when f(x) = x + 5 and g(x) = 5x2 + 10x - 5.

Answer 1

f g = ( x + 5 ) · ( 5 x² + 10 x - 5 ) =
= 5 x³ + 10 x² - 5 x + 25 x² + 50 x - 25 = 5 x³ + 35 x² + 45 x - 25 =
= 5 ( x³ + 7 x² + 9 x - 5 )
(f g) ( - 4 ) = 5 * ( - 64 + 112 - 36 - 5 ) = 5 * 7 = 35