Math Is Fun Forum

numquester

Member

Registered: 2016-12-12

Posts: 19

The quadratic formula

I came across a video in which the author explains visually how to arrive at the quadratic formula, except he skips some steps.

Here is the video:  https://www.youtube.com/watch?v=EBbtoFMJvFc

I follow it up to this point, where some steps are skipped:  https://www.youtube.com/watch?v=EBbtoFMJvFc&t=7m45s

In the video, the right side of the equation goes from: 

To: 

So I think that I can rewrite the first example like this: 

But I don't see how the a can be moved to the numerator.  Or maybe I'm approaching it all wrong.

By the way, I really like this approach to explaining math.  I never understood what 'completeing the square' actually means until I saw this video.  It seems right at home with so many of the nice explanations here at Math is Fun.

Side note:  I really appreciate the LaTeX integration in the forum.  This is nice!

Last edited by numquester (2017-01-10 16:49:17)

Bob

Administrator

Registered: 2010-06-20

Posts: 10,626

Re: The quadratic formula

hi numquester

Think about how you would add two fractions.

This is what you have done:

This isn't correct.  You have to put both fractions over a common denominator and then just add the tops:

So in the formula  multiply the first fraction top and bottom by 4a

Hopefully you can see how this works now.

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

numquester

Member

Registered: 2016-12-12

Posts: 19

Re: The quadratic formula

Bob, that is a very clear explanation.  Thanks for taking the time.