Homework Help: math

Posted by carol on Wednesday, May 4, 2011 at 4:02pm.

In solving the equation (x + 4)(x – 7) = -18, Eric stated that the solution would be
x + 4 = -18 => x = -22
or
(x – 7) = -18 => x = -11
However, at least one of these solutions fails to work when substituted back into the original equation. Why is that? Please help Eric to understand better; solve the problem yourself, and explain your reasoning.

math - Reiny, Wednesday, May 4, 2011 at 6:14pm
Eric does not understand the concept

It only works if the product is zero
so expand ....

x^2 - 3x - 28 = -18
x^2 - 3x - 10 = 0
(x-5)(x+2) = 0
x = 5 or x = -2

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