1.

For what values of a and b so that the polynomial x3 + 10x2 + ax – 6 is exactly divisible by (x – 1) and (x + 2).

Answer»

Let p(x) = x3 + 10x2 + ax + b

p(x) i.e. x3 + 10x2 + ax + 6 is exactly divisible by (x – 1) and (x + 2)

Therefore, p(1) and p(-2) must equal to zero.

p(1) = (1)3 + 10(1)2 + a x 1 + b = 0

⇒ 1 + 10 + a + b = 0

⇒ a + b = – 11 …(i)

Also, p(-2) = 0

(-2)3 + 10(-2)2 + a x (- 2) + b = 0

-8 + 40 – 2a + b = 0

⇒ – 2a + b = – 32 …(ii)

Solving (i) and (ii), we get

a = 7 and b = -18.



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