i have to show this equation represents a sphere..

but i'm stuck on the z variable as it won't come down to an even square. it comes down to a (x+2)(x-2).

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- October 1st 2010, 05:19 AMgreencheesecaequation of a sphere
i have to show this equation represents a sphere..

but i'm stuck on the z variable as it won't come down to an even square. it comes down to a (x+2)(x-2). - October 1st 2010, 05:20 AMmr fantastic
- October 1st 2010, 05:31 AMgreencheeseca
- October 1st 2010, 08:18 AMgreencheeseca
so, what can i do about the z variable? i can't get it to work out as an even square..

- October 1st 2010, 09:50 AMearboth
- October 1st 2010, 01:34 PMmr fantastic
- October 2nd 2010, 02:44 AMHallsofIvy
You apparently are trying to do this as

which is wrong for two reasons. First, moving that "- 4" to the other side of the equation makes it " " which cannot be factored, not " ".

But you**don't**want to do that to begin with!

Instead leave that constant, 4, on the right and do it as

Complete the square in x and y. The "z" term is**already**a perfect square! The numbers you need to add on the left to make the perfect squares, and, of course, add on the right, will give you a positive number on the right.