| If you want to easily write any quadratic function in vertex form, just use these formulas:
f(x)=ax^2 +bx+c
if a>0, then:
f(x)=(x+(b/2))^2 −((b/2))^2 +c
if a<0, then:
f(x)=−(x−((b/2)))^2 +((b/2))^2 +c
Let′s take some examples:
f(x)=x^2 −6x+7
f(x)=(x−(6/2))^2 −((6/2))^2 +7
f(x)=(x−3)^2 −9+7
f(x)=(x−3)^2 −2
Now let′s take the same example, but when a<0:
f(x)=−x^2 −6x+7
f(x)=−(x−(−(6/2)))^2 +((6/2))^2 +7
f(x)=−(x+3)^2 +9+7
f(x)=−(x+3)^2 +16
Another example:
f(x)=x^2 +4x−5
f(x)=(x+(4/2))^2 −((4/2))^2 −5
f(x)=(x+2)^2 −4−5
f(x)=(x+2)^2 −9
Now let′s also see what happens when a<0:
f(x)=−x^2 +4x−5
f(x)=−(x−((4/2)))^2 +((4/2))^2 −5
f(x)=−(x−2)^2 +4−5
f(x)=−(x−2)^2 −1
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