SOLUTION: Please help me solve: 3x^2-12x-13=0 by completing the square.

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Question 587558: Please help me solve: 3x^2-12x-13=0 by completing the square.
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
+3x%5E2-12x-13=0+
Add 13 to both sides
+3x%5E2+-+12x+=+13+
Divide both sides by 3
+x%5E2+-+4x+=+13%2F3+
Take 1/2 of the coefficient of x,
square it, and add it to both sides
+x%5E2+-+4x+%2B+%28%28-4%29%2F2%29%5E2+=+13%2F3+%2B+%28%28-4%29%2F2%29%5E2+
+x%5E2+-+4x+%2B+%28-2%29%5E2+=+13%2F3+%2B+%28-2%29%5E2+
+x%5E2+-+4x+%2B+4+=+13%2F3+%2B+4+
+x%5E2+-+4x+%2B+4+=+13%2F3+%2B+12%2F3+
+x%5E2+-+4x+%2B+4+=+25%2F3+
+%28+x+-+2+%29%5E2+=+%28+5%2Fsqrt%283%29%29%5E2+
Take the square root of both sides
+x+-+2+=+5%2Fsqrt%283%29+
+x+=+2+%2B+5%2Fsqrt%283%29+
and also
+x+-+2+=+-5%2Fsqrt%283%29+
+x+=+2+-+5%2Fsqrt%283%29+
These are the 2 solutions
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I will check by using approximate values
+x+=+2+%2B+5%2F1.732+
+x+=+2+%2B+2.8868+
+x+=+4.8868+
Plugging this back into original equation
+3x%5E2-12x-13=0+
+3%2A4.8868%5E2-12%2A4.8868-13=0+
+3%2A23.8808+-+58.6416+-+13+=+0+
+71.6424+-+58.6416+-+13+=+0+
+.0008+=+0+
I believe the error is due to rounding off
You can check the other solution