If both (x+1) and (x -1) are factors of ax3 + x2 - 2x + b , find a and b. -Maths 9th

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Answer :

Let p(x) = ax3 + x2 - 2x + b  Since (x+1) and (x-1) are the factors of p(x), ∴ p(-1) = 0 and p(1) = 0 ∴ p(-1) = a(-1)3 + (-1)2 - 2  (-1) + b = 0 ⇒ - a + 1 + 2 + b = 0  ⇒ a - b = 3 ---- (i)  Again, p(1) = a(1)3 + (1)2 - 2(1) + b = 0 ⇒ a + 1 - 2 + b = 0 ⇒ a + b = 1 ----- (ii)  solving equations (i) and (ii) we get a = 2 and b = -1     

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