Mathematics
Find the values of constants a and b when x - 2 and x + 3 both are the factors of expression x3 + ax2 + bx - 12.
Factorisation
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Answer
x - 2 = 0 ⇒ x = 2
Since, x - 2 is a factor of x3 + ax2 + bx - 12
∴ On substituting x = 2 in x3 + ax2 + bx - 12, remainder = 0.
⇒ (2)3 + a(2)2 + b(2) - 12 = 0
⇒ 8 + 4a + 2b - 12 = 0
⇒ 4a + 2b - 4 = 0
⇒ 4a + 2b = 4
⇒ 2(2a + b) = 4
⇒ 2a + b = 2
⇒ b = 2 - 2a ………(i)
x + 3 = 0 ⇒ x = -3
Since, x + 3 is a factor of x3 + ax2 + bx - 12
∴ On substituting x = -3 in x3 + ax2 + bx - 12, remainder = 0.
⇒ (-3)3 + a(-3)2 + b(-3) - 12 = 0
⇒ -27 + 9a - 3b - 12 = 0
⇒ 9a - 3b - 39 = 0
⇒ 9a - 3b = 39
⇒ 3(3a - b) = 39
⇒ 3a - b = 13
⇒ b = 3a - 13 ………(ii)
From (i) and (ii) we get,
⇒ 2 - 2a = 3a - 13
⇒ 3a + 2a = 2 + 13
⇒ 5a = 15
⇒ a = 3.
Substituting value of a in (i),
⇒ b = 2 - 2a = 2 - 2(3) = 2 - 6 = -4.
Hence, a = 3 and b = -4.
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