How do you write y=-x^2+20x-80 in vertex form?

2 Answers
Apr 26, 2017

y=-(x-10)^2+20" " is the vetex form.

Explanation:

The vertex form of a quadratic equation is :
" "
color(blue)(y=a(x-h)^2+k)" " where (h,k) " "is the vertex
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The quadratic form is performed by factorization.
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y=-x^2+20x-80
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rArry=-(x^2-20x+80)
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rArry=-((x)^2-2(x)(10)+80)
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To complete the square of the above equation we recognize that the second term should be color(red)(10^2=100 so we should add 20
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rArry=-((x)^2-2(x)(10)+80color(red)(+20-20))
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rArry=-((x)^2-2(x)(10)+100-20)
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rArry=-((x)^2-2(x)(10)+100)+20
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rArry=-((x)^2-2(x)(10)+10^2)+20
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rArry=-(x-10)^2+20
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Therefore,y=-(x-10)^2+20" " is the vertex form of the parabola where " "(10,20) is its vertex.

Apr 26, 2017

y = -(x-10)^2 +20

Explanation:

To write a quadratic trinomial in vertex from you need to change:

y = ax^2 +bx +c " " into the form " "y = p(x+q)^2 + t

In y = p(x+q)^2 + t the vertex is at (-q, t)

y = -x^2 +20x-80" "larr make x^2 positive

y = -[x^2 -20x+80]" "larr complete the square

y = -[x^2 -20x color(blue)(+ 100-100)+80]" "larrcolor(blue)(+(b/2)^2 -(b/2)^2)

y =-[(x-10)^2 -20]

y = -(x-10)^2 +20

This is now vertex form, giving the vertex as (10,20)