If is exactly divisible by , then find the value of ab.
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Let p ( x ) = a x 3 + b x − c Let g ( x ) = x 2 + b x + c Now, when forming the polynomial g(x) , the constant was k = 1 But putting k = a will not change g(x)'s roots => g ( x ) = a ( x 2 + b x + c ) => g ( x ) = a x 2 + a b x + a c
Now dividing p(x) by g(x) will give us q ( x ) = x − b And p(x) divided by q(x) will give us g(x). But while taking the quotient we find that we get g ( x ) = a x 2 + a b x + b ( a b + 1 ) and remainder = b 2 ( a b + 1 ) − c = 0 Equating the two expressions of g(x) we get b ( a b + 1 ) = a c − − − − − ( 1 ) From remainder; b 2 ( a b + 1 ) − c = 0 => b × b ( a b + 1 ) = c => b × a c = c f r o m ( 1 ) => a b = 1 Q.E.D