SOLUTION: Is the following trinomial a perfect square? why or why not x^2+18x-81

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Question 119955: Is the following trinomial a perfect square? why or why not
x^2+18x-81
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
It can't be because of the (-) sign in front of the 81
The perfect square can only have two forms
%28x+-+a%29%5E2+=+0
or
%28x+%2B+a%29%5E2+=+0
The 1st gives me
x%5E2+-+2a+%2B+a%5E2+=+0
and the 2nd gives me
x%5E2+%2B+2a+%2B+a%5E2+=+0
Notice that the sign in front of the constant term, a%5E2 is
(+) in both cases