a = - 4 and b = 5
complex roots occur in conjugate pairs, hence
if 2 + i is a root then 2 - i is also a root and
(x - (2 + i) )(x - (2 - i) ) are the factors
= (x - 2 - i)(x - 2 + i)
= (x - 2)² - i² ( i² = - 1 )
= x² - 4x + 4 + 1
= x² - 4x + 5
compare the coefficients of x² + ax + b with x² - 4x + 5
⇒ a = - 4 and b = 5
