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# How do you factor an equation like ax^2+bx+c if the a is negative and there is no c?

I know how to graph these kinds of equations with parabolas but cannot figure out how to find the factored, vertex, and standard forms of these equations.

### 2 Answers by Expert Tutors

Jason S. | My goal is the success of my students. Knowledge-Patience-HonestyMy goal is the success of my students. K...
4.9 4.9 (115 lesson ratings) (115)
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Like this?

y = -x^2 + 4x

If so,

Standard form = -(x^2) + 4x

For vertex form,

Factor out the - sign to get the reflection/stretch (this is "a" in y=a(x-h)^2 + k

y = -(x^2 - 4x)

Complete the square:

y = -(x^2 - 4x + 4) + 4   (adding NEGATIVE 4 inside to complete the square. Do not forget the - factored outside the parentheses.  Since you added -4 to complete the square, you must add +4 outside to have a net effect of nothing being added or subtracted on that side of the equation.

y = -(x-2)^2 + 4

Vertex = (2,4)

Marvin F. | One of the Best Physics, Chemistry, Math and Academic Skills TutorsOne of the Best Physics, Chemistry, Math...
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Don't get confused by the fact that a is negative and c=0.  This is standard factoring.

ax^2+bx+c = (ax+b)(x)     Remember: c=0