## What rule do I break here?

Simplificatation, evaluation, linear equations, linear graphs, linear inequalities, basic word problems, etc.
Jherek2
Posts: 9
Joined: Sat Aug 04, 2012 2:45 pm
Contact:

### What rule do I break here?

I'm factoring the difference of two cubes: mx3+my3-x-y,
I get this far
m(x+y)(x2+xy+y2)-x-y
and then I do this
m(x+y)(x2+xy+y2)-1(x+y) to get (m-1)(x+y)(x2+xy+y2) which is wrong.

I've worked out what I should have done (I think!),
(x+y)m(x2+xy+y2)-x-y
(x+y)(mx2+mxy+my2)-x-y
(x+y)(mx2+mxy+my2)-1(x+y)
(x+y)(mx2+mxy+my2-1)

The worrying thing is, I don't know WHY I can't do what I did when I went wrong ! So how do I recognise when I'm going to do it again?

stapel_eliz
Posts: 1628
Joined: Mon Dec 08, 2008 4:22 pm
Contact:
I'm factoring the difference of two cubes: mx3+my3-x-y,
I get this far
m(x+y)(x2+xy+y2)-x-y
and then I do this
m(x+y)(x2+xy+y2)-1(x+y) to get (m-1)(x+y)(x2+xy+y2) which is wrong....

The worrying thing is, I don't know WHY I can't do what I did when I went wrong
Try highlighting things in excruciating clarity...

. . . . .m(x + y)(x2 + xy + y2) - x - y

. . . . .m(x + y)(x2 + xy + y2) - 1(x + y)

Can you see now that the only thing that factors out front is the (x + y), leaving the m(x2 + xy + y2) - 1 to be combined and simplified inside the parentheses?

Jherek2
Posts: 9
Joined: Sat Aug 04, 2012 2:45 pm
Contact:

### Re: What rule do I break here?

Thank you for the explanation . When someone shows me something I'm stuck on, it always seems so easy! I need to slow down a bit and get more practice!