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III (a+b)(a-b) = a2-b2This is one of the recycled quadratics - also known as the difference of squares. It is GMAC's favorite, and appears in many questions in either of its forms. Whichever of the above forms you encounter in a GMAT problem, changing the quadratic into the other form will usually set you on the right path to solving the problem quickly. The trick lies in identifying the recycled quadratic in both its forms:1) Factored form: (a+b)(a-b)a. Identification:Two sets of parentheses, one with the sum of two terms, the other with the difference of the same two terms. Example:(x+5)(x-5)b. First step to solution: change the form: turn it into expanded form - square a and b, and subtract the square of b from the square of a. Check that it works using FOIL:(x+5)(x-5) = x·x + x·-(5) + 5x + 5·(-5) = x2-52 = x2-25

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