Mathematics
If x2 - 3x + 2 is a factor of x3 - ax2 + b, find the values of a and b.
Factorisation
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Answer
Factorising, x2 - 3x + 2.
⇒ x2 - 2x - x + 2
⇒ x(x - 2) - 1(x - 2)
⇒ (x - 1)(x - 2).
So, we can say (x - 1) and (x - 2) are the factors of x3 - ax2 + b.
By factor theorem,
A polynomial f(x) has a factor (x - a), if and only if, f(a) = 0.
Given, 1st factor
⇒ x - 1 = 0
⇒ x = 1.
Substituting x = 1 in x3 - ax2 + b, will give remainder = 0.
⇒ 13 - a(1)2 + b = 0
⇒ 1 - a + b = 0
⇒ a = b + 1 ………(Eq. 1)
Given, 2nd factor
⇒ x - 2 = 0
⇒ x = 2.
Substituting x = 2 in x3 - ax2 + b, will give remainder = 0.
⇒ 23 - a(2)2 + b = 0
⇒ 8 - 4a + b = 0
⇒ 4a = b + 8
Substituting value of a from Eq 1 in above equation :
⇒ 4(b + 1) = b + 8
⇒ 4b + 4 = b + 8
⇒ 4b - b = 8 - 4
⇒ 3b = 4
⇒ b =
⇒ a = b + 1 = .
Hence, a = and b = .
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