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).
Completing the square by factoring out a negative one
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Questions answered by this video:
How do I complete the square when there is a negative sign in front of the x^2 term?
How do I get a quadratic equation that is in standard form into vertex form?
How do I find the zeros of a quadratic function?
How do I complete the square on the function f(x)=-x^2+4x+1?
How do I get -x^2+4x+1 into vertex form?
What are the zeros of the function f(x)=-x^2+4x+1?
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This lesson teaches how to complete the square on a quadratic function which gets the equation into vertex form. Then he shows how to find the zeros of the function once you have it in vertex form.