a + b = 7
ab = -18
Find a^3 + b^2
Square both sides of (1) a^2 + 2ab + b^2 = 49 ⇒ a^2 + b^2 = 49 - 2ab
a^3 + b^3 = (a + b) (a^2 - ab + b^2)
Subbing....we have that
a^3 + b^3 = ( 7) ( a^2 + b^2 - ab)
a^3 + b^3 = ( 7) (49 - 2ab - ab)
a^3 + b^3 = (7) (49 - 3ab)
a^3 + b^3 = (7) (49 - 3(-18) )
a^3 + b^3 = 7 ( 49 + 54)
a^3 + b^3 = 7 ( 103)
a^3 + b^3 = 721