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Algebra Level 3

A polynomial g ( x ) = x 3 + a x 2 + b x + c g(x) = x^3 + ax^2 + bx + c has 3 3 distinct roots and each root of g ( x ) = 0 g(x) = 0 is also a root of f ( x ) = x 4 + x 3 + b x 2 + 100 x + c f(x) = x^4 + x^3 + bx^2 + 100x + c .

Then find the value of f ( 1 ) f(1) .


The answer is 202.

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1 solution

Ravneet Singh
Aug 29, 2017

Let the additional root of f ( x ) f(x) be λ \large \lambda .

Using Vieta's Formula, product of roots of g ( x ) g(x) is c \large -c and product of roots of f ( x ) f(x) is c \large c .

c × λ = c λ = 1 \therefore \quad \large -c \times \lambda = c \implies \lambda = -1

Also, Using Vieta's Formula, sum of roots of g ( x ) g(x) is a \large -a and sum of roots of f ( x ) f(x) is 1 \large -1 .

a + λ = 1 a = 0 \therefore \quad \large -a + \lambda = -1 \implies a = 0

hence f ( x ) = ( x + 1 ) g ( x ) \large f(x) = (x+1)g(x)

f ( 1 ) = 2 g ( 1 ) \large \therefore f(1) = 2g(1)

102 + b + c = 2 × ( 1 + b + c ) \implies \large 102 + b + c = 2 \times (1 + b + c)

b + c = 100 \large \implies b + c = 100

f ( 1 ) = 2 × ( 1 + 100 ) = 202 \large f(1) = 2 \times (1 + 100) = \boxed{202}

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