SOLUTION: How do I complete The square ofthis trinomial xsqauared-4x-21=0

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Question 419891: How do I complete The square ofthis trinomial xsqauared-4x-21=0
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
+x%5E2+-+4x+-+21+=+0+
x%5E2+-+4x+=+21+
Take 1/2 of the coefficient of x, square it,
then add it to both sides
x%5E2+-+4x+%2B+%284%2F2%29%5E2+=+21+%2B+%284%2F2%29%5E2+
+x%5E2+-+4x+%2B+4+=+21+%2B+4+
Now both sides are perfect squares
+%28x+-+2%29%5E2+=+5%5E2+
Take the square root of both sides
+x+-+2+=+5
+x+=+7+
also,
+x+-+2+=+-5+
x+=+-3+
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so, x+-+7+=+0 and x+%2B+3+=+0+ and
+x%5E2+-+4x+-+21+=+%28x+-+7%29%2A%28x+%2B+3%29+