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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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