Teacher2Teacher Q&A #104

Teachers' Lounge Discussion: Factoring trinomials

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From: Thunder

To: Teacher2Teacher Public Discussion
Date: 2002090221:48:53
Subject: Factoring 35x^2-88x+45

To factor 35x^2 - 88x + 45 List the factors of the 'a' term vertically: 1 5 7 35 35 7 5 1 List the factors of the 'c' term vertically: 1 3 5 45 15 9 Now cross multiply each set of 'a' factors with each set of 'c' factors. Since 'c' is positive, you ADD these two products to find the sum of 88. I use a giant T to visualize. (If 'c' were negative, you would SUBTRACT the two products.) a = 35 | c = 45 --------------|------------- 1 5 7 35 | 1 3 5 35 7 5 1 | 45 15 9 -----|----- In this case we use the factors: 7 | 5 5 | 9 7 x 9 = 63 5 x 5 = 25 ---- 88 Now you read the factors across the T as such: (7x - 5)(5x - 9) Since the 'b' term is negative, the second term in each binomial is also negative.

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