Question 878264
In {{{x^2-8x+12}}}
The first two terms, {{{x^2-8x}}} ,
remind you of
{{{(x-4)^2}}} .
Your error is that
{{{(x-4)^2=x^2-8x+4^2=x^2-8x+16}}} .
So {{{x^2-8x+12=x^2-8x+16-16+12=(x-4)^2-16+12=(x-4)^2-4}}}
You forgot to square the {{{4}}} ,
as if {{{(x-4)^2}}} was {{{x^2-8x+4}}} .