高手請入~ A math

1. Let f(x) = ax^3+8x^2+bx+6
x^2-2x-3 = (x-3)(x+1)
Using the remainder theorem,
f(3) = 0
a(3^3)+8(3^2)+3b+6 = 0
27a+72+3b+6 = 0
27a+3b+78 = 0
9a+b+26 = 0 ---(1)
f(-1) = 0
a(-1)^3+8(-1)^2-b+6 = 0
-a+8-b+6 = 0
-a-b+14 = 0 ---(2)
(1)+(2): 8a+40 = 0
8a = -40
a = -5
sub a = -5 into (2):
-(-5)-b+14 = 0
-b = -19
b = 19
2. Let g(x) = x^3+Ax-12
g(-3) = 0
(-3)^3-3A-12 = 0
-27-3A-12 = 0
-3A = 39
A = -13

Q.1就唔係幾識...
Q.2因為條式講明佢係可以被整除,,所以你計到果個REMAINDER係等於O

x^3+Ax-12/ x+3
=X^2-3-A-7....REMAINDER 係12+3A+27

因為REMAINDER係等於O
所以
12+3A+27=O
3A+39=O
A=-13