Seemingly complex

Algebra Level 3

Find the value of x x for which the below given expression has a solution

x 2 + x 2 x 2 x 2 = x + 2 x 2 \frac { x^{2} +x -2} { x^{2}-x-2}= \frac {x+2}{x-2}


The answer is -2.

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2 solutions

John Mead
Jun 16, 2014

Factor the quadratics on the left hand side:

x 2 + x 2 x 2 x 2 = ( x + 2 ) ( x 1 ) ( x 2 ) ( x + 1 ) \frac{x^2+x-2}{x^2-x-2}=\frac{(x+2)(x-1)}{(x-2)(x+1)}

We substitute and see

( x + 2 ) ( x 1 ) ( x 2 ) ( x + 1 ) = x + 2 x 2 \frac{(x+2)(x-1)}{(x-2)(x+1)}=\frac{x+2}{x-2}

This equation takes the form a b = a ab=a , where a = x + 2 x 2 a=\frac{x+2}{x-2} and x 1 x + 1 = b \frac{x-1}{x+1}=b .

This form of equation has solutions for b = 1 b=1 or a = 0 a=0 . (Verify by subtracting a a , factoring and applying the zero product property.)

But if b = x 1 x + 1 = 1 b=\frac{x-1}{x+1}=1 , x 1 x-1 and x + 1 x+1 , which differ by 2, must be equal, b = 1 b=1 does not lead to any solution.

Otherwise we consider a = x + 2 x 2 = 0 a=\frac{x+2}{x-2}=0 , which holds if the numerator, x + 2 = 0 x+2=0 , giving x = 2 \boxed{x=-2} as the unique solution.

敬全 钟
Jun 16, 2014

First, cross multiplying to get ( x 2 ) ( x 2 + x 2 ) = ( x + 2 ) ( x 2 x 2 ) (x-2)(x^2+x-2)=(x+2)(x^2-x-2) Simplifying a bit to get 2 x 2 8 = 0 2x^2-8=0 2 ( x 2 ) ( x + 2 ) = 0 2(x-2)(x+2)=0 Then, we have x = 2 x=2 or x = 2 x=-2 as our solutions. Notice that if we substitute x = 2 x=2 to the equation, the RHS will become 4 0 \frac{4}{0} which is undefined. Thus, the only solution to this equation is x = 2 x=\boxed{-2} .

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