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 Algebra Pre-Algebra and Basic Algebra Math Forum

May 20th, 2015, 03:07 AM   #11
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Quote:
 Originally Posted by skipjack $\displaystyle \frac{x^2 - 121}{x^2 - 144} \cdot \frac{3x + 36}{2x + 22} = \frac{(x - 11)(x + 11)}{(x - 12)(x + 12)} \cdot \frac{3(x + 12)}{2(x + 11)} = \frac{3(x - 11)}{2(x - 12)}$
thanks what happens with the numbers 121 and 144, why do they disappear?

 May 20th, 2015, 07:27 PM #12 Senior Member     Joined: Nov 2010 From: Indonesia Posts: 2,001 Thanks: 132 Math Focus: Trigonometry It's because $\displaystyle x^2-a^2 = (x+a)(x-a)$, thus: $\displaystyle x^2-121 = x^2-11^2 = (x+11)(x-11)$ and $\displaystyle x^2-144 = x^2-12^2 = (x+12)(x-12)$

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