The general Formula looks like this:
\(x^2 +bx + ( \frac{b}{2})^2\)
This really only involves 2 steps:
1) Take half of "x" coefficient. ----> -8/2 = -4
2) Square that number. ----------> (-4)^2 = 16
Perfect trinomial for this example is then:
\(x^2 -8x+16\)
