'Rooty' polynomial!

Algebra Level 3

If a , b , c a , b , c are the distinct roots of x 3 27 x 2 3081 x 3053 = 0 x^3 - 27x^2 - 3081x - 3053 = 0 , such that a > b > c a > b > c .

Find 2 a + 7 b + c 2a + 7b + c .


The answer is 92.

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2 solutions

Nihar Mahajan
Feb 19, 2015

Given polynomial : x 3 27 x 2 3081 x 3053 = 0 x^3 - 27x^2 - 3081x - 3053 = 0

By possible rational root theorem , we can easily figure out that 1 -1 is a root of this polynomial.

Thus, By Factor theorem , ( x + 1 ) (x + 1) is a factor of x 3 27 x 2 3081 x 3053 = 0 x^3 - 27x^2 - 3081x - 3053 = 0 .

Thus by Division algorithm the given polynomial can be written as :

x 3 27 x 2 3081 x 3053 = ( x + 1 ) ( x 2 28 x 3053 ) = 0 x^3 - 27x^2 - 3081x - 3053 = (x + 1)(x^2 - 28x - 3053) = 0

Since we have already got 1 -1 as a root ,

( x 2 28 x 3053 ) = 0 (x^2 - 28x - 3053) = 0

( x 71 ) ( x + 43 ) = 0 (x - 71)(x + 43) = 0 This gives us that 71 , 43 71 , -43 are the other roots of the polynomial.

Thus , a = 71 , b = 1 , c = 43 2 a + 7 b + c = 2 ( 71 ) + 7 ( 1 ) + ( 43 ) = 92 a = 71 , b = -1 , c = -43 \Rightarrow 2a + 7b + c = 2(71) + 7(-1) + (-43) =\huge\boxed{92}

Very neat solution !

Venkata Karthik Bandaru - 6 years, 3 months ago

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Thank you!

Nihar Mahajan - 6 years, 3 months ago

But is there any easy way method to factoRise

x^{2} -28x-3053

Abhisek Mohanty - 6 years, 2 months ago
Irtaza Sheikh
Apr 15, 2015

Very easy solution is that Take your casio calculator .press MODE and select eq.then tell all co-efficient and in the last press equal(=) . You ll get these 3 Ans.. but that will be a big loss for you that you cant approaches to ans without calculator... I done it by both to verify myself either Am I right Or Am I right??

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