Mathematics
If (x + 2) and (x + 3) are factors of x3 + ax + b, find the values of a and b.
Factorisation
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Answer
Let f(x) = x3 + ax + b
Since (x + 2) and (x + 3) are factors, by the factor theorem, f(−2) = 0 and f(−3) = 0.
⇒ f(-2) = 0
⇒ (-2)3 + a(-2) + b = 0
⇒ -8 - 2a + b = 0
⇒ -2a + b = 8 ….(1)
⇒ f(-3) = 0
⇒ (-3)3 + a(-3) + b = 0
⇒ -27 - 3a + b = 0
⇒ -3a + b = 27 ….(2)
Subtract equation (2) from equation (1), we get:
⇒ -2a + b - (-3a + b) = 8 - 27
⇒ -2a + 3a = -19
⇒ a = -19
Substituting value of a in equation (1), we get :
⇒ -2(-19) + b = 8
⇒ 38 + b = 8
⇒ b = 8 - 38
⇒ b = -30
Hence, the value of a = -19 and b = -30.
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