maths!!!!!

Let f(x) = x3– px2 + qx – 18
By factor theorem, f(-2)= 0
i.e. (-2)3 –p(-2)2 + q(-2) – 18 = 0
-8 – 4p – 2q – 18 = 0
2p + q = -13 ─── (1)
By factor theorem, f(3)= 0
i.e. (3)3 –p(3)2 + q(3) – 18 = 0
27 – 9p + 3q – 18 = 0
3p – q = 3 ─── (2)
(1) X 3 – (2) X 2:
5q = -45
q = -9
p = 4

2008-04-21 07:18:39 補充：
2p + q = -13 ─── (1)
3p – q = 3 ─── (2)
(1) + (2):
5p = -10
p = -2
q = -9