# Complete the Square??

• Oct 30th 2007, 07:39 PM
B1GG13
Complete the Square??
Hello,
Can someone please explain to me how to solve this problem.

Complete the square x^2-8x

I keep looking over the explanation in my book and i just doesn't make sense to me.

Thanks :confused:
• Oct 30th 2007, 08:32 PM
ThePerfectHacker
x^2 - 8x = (x^2 - 8x + 16 )- 16 = (x-4)^2 - 16
• May 10th 2015, 03:09 PM
TheVirtualMathematician
Re: Complete the Square??
You should know that $(x- a)^2= x^2- 2ax+ a^2$. Comparing that to $x^2- 8x$ you can see that "2ax" must be "8x" so that 2a= 8, a= 4. And then $a^2= 4^2= 16$. To make $x^2- 8x$ into the "perfect square" $x^2- 8x+ 16$ we need to add 16- and, of course, in order not to change the value we must also subtract 16: $x^2- 8x+ 16- 16= (x- 4)^2- 16$.