If (x + 2) is a factor of p (x) = ax3 + bx2 + x – 6 and p(x) when divided by (x – 2) leaves a remainder 4, prove that a = 0 and b = 2.
We have,
p (x) = ax3 + bx2 + x – 6
It is given in the question that, (x + 1) is a factor of p (x)
x + 2 = 0
x = - 2
∴ f (-2) = 0
a (-2)3 + b (-2)2 + (-2) – 6 = 0
- 8a + 4b – 2 – 6 = 0
- 8a + 4b – 8 = 0
- 4 (2a – b + 2) = 0
2a – b + 2 = 0 (i)
Also, it is given in the question that when p (x) is divided by (x – 2) then it leaves a remainder 4
∴ p (2) = 4
a (2)3 + b (2)2 + (2) – 6 = 4
8a + 4b + 2 – 6 = 4
8a + 4b – 4 - 4= 0
4 (2a + b – 2) = 0
2a + b – 2 = 0 (ii)
Now, adding (i) and (ii) we get:
2a – b + 2 + 2a + b – 2 = 0
4a = 0
a = 0
Putting the value of a in (ii), we get:
2 (0) + b – 2 = 0
b – 2 = 0
b = 2
Hence, it is proved that the value of a is 0 and that of b is 2