Are You Good With Modulus?

Algebra Level 3

x 2 + x 4 = x \large x^2+x-4=|x|

If the sum of all roots to the equation above equals to a b a - \sqrt b for some positive integer a a and b b , find the value of a + b a+ b .


The answer is 6.

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1 solution

Rachit Shukla
Apr 6, 2015

Equation: x 2 + x x 4 = 0 x^2+x-|x|-4=0

CASE I: If x 0 x\geq0 , then equation becomes x 2 + x x 4 = 0 x^2+x-x-4=0

\Rightarrow x = ± 2 x=\pm2

So, x = 2 x=2 is a solution in this case.

CASE II: If x < 0 x<0 , then equation becomes x 2 + x ( x ) 4 = 0 x^2+x-(-x)-4=0

\Rightarrow x 2 + 2 x 4 = 0 x^2+2x-4=0

\Rightarrow x = 1 ± 5 x=-1\pm\sqrt{5}

So, x = 1 5 x=-1-\sqrt{5} is a solution in this case.

Therefore, S u m = 2 1 5 Sum=2-1-\sqrt{5}

\Rightarrow 1 5 1-\sqrt{5}

\Rightarrow a + b = 1 + 5 = 6 a+b=1+5=6 ¨ \ddot\smile

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