# Completing the Square by Factoring Out -1

Taught by mrbrianmclogan
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Part of video series
Meets NCTM Standards:
Errors in this video:

At about the 0:34 mark, the teacher says our goal is to get it into...standard form; he should have said get it into vertex form.

At about the 4:42 mark, the teacher writes Â±sqrt(5)=x+2; he should have written Â±sqrt(5)=x-2. Because of this, he also gets the wrong zeros; he should get x=2Â±sqrt(5).

Lesson Summary:

This lesson teaches completing the square by factoring out -1. After isolating the x-terms and making it into a binomial square, students can add and subtract the constant value inside the trinomial, which can be multiplied by the -1 factor. This will provide a perfect square trinomial that can be written in binomial square format. Through solving for the zeros, students can determine the value of x, and avoid common mistakes such as not factoring out the negative one or forgetting to divide it first before taking the square root.

Lesson Description:

Completing the square by factoring out a negative one

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