Misalkan a ≠ 0, serta x dan y adalah akar-akar x2-(a+1/a)x+1=0

x² -(a+(1/a)x + 1 = 0

berarti :

x + y = a + (1/a)

x + y = (a²+1)/a

xy = 1

x² + bx + c = 0

berarti

2x + 2y = -b

2(x+y) = -b

2(a²+1)/a = -b

(2a² + 2)/a = -b

2a² + 2 = -ab

2a² + ab = -2

2x. 2y = c

4xy = c

4(1) = c

4 = c

jadi :

2a² + c + ab =

2a² + ab + c =

-2 + 4 =

2