Simplify:

(a + 3b)(a + 5b) - (a + 4b)^2

My steps:

- parenthesis first, then exponent

a^2 + 5ab + 3ab + 15b^2 - a^2 + 16b^2

- add/subtract like terms

31b^2 + 8ab

The answer key given is -b^2. Where did I go wrong?

Thanks.

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- December 4th 2006, 11:13 PM #1

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- December 4th 2006, 11:21 PM #2

- December 5th 2006, 12:28 PM #3