Answer:
Step-by-step explanation:
let the quadratic eq. be ax²+bx+c=0
or x²+b/a x+c/a=0
or x²+b/ax=-c/a
to complete the square add both sides (b/2a)²or b²/4a²
x²+b/a x+(b/2a)²=b²/4a² -c/a
(x+b/2a)²=(b²-4ac)/(4a²)
taking square root
[tex]x+\frac{b}{2a} =\pm \frac{\sqrt{b^2-4ac} }{2a} \\or\\x=-\frac{b}{2a} \pm\frac{\sqrt{b^2-4ac} }{2a} \\or\\x=\frac{-b \pm\sqrt{b^2-4ac} }{2a}[/tex]
now you must know why -b and not +b
