(tiếp)
theo hệ thức viet , ta có
S = x1 + x2 = -b/a = 3
P = x1.x2 = c/a = 1
d) x1^3 + x2^3
= (x1 + x2)^3 - 3x1.x2(x1 + x2)
= S^3 - 3P.S
= 3^3 - 3.1.3
= 27 - 9
= 18
e) x1^4 + x2^4
= (x1^2 + x2^2)^2 - 2(x1.x2)^2
= [(x1 +x2)^2 - 2x1.x2]^2 - 2(x1.x2)^2
= (S^2 - 2P)^2 - 2P^2
= (3^2 - 2.1)^2 - 2.1^2
= (9 - 2)^2 - 2
= 49 - 2
= 47