factoring: 4r^2-12rs-9s^2 (multiples of 9 adding to -12)

[ayva00]

I'm stuck in this problem, 4r^2-12rs-9s^2, i'm not sure where to begin :/ because normally i would think multiples of 9 that add up to -12, but i cant think of any.
Thanks for your help

 

[mmm4444bot]

… normally i would think multiples of 9 that add up to -12, …

I think that you mean "factors of 9".

Yes, that's the normal line of reasoning when the coefficient on the first term is 1.

This is not the case, with your exercise. Because the first term has a coefficient of 4, you first need to multiply the 4 and -9 together.

4(-9) = -36.

Now, look for factors of -36 that add up to -12.

Then, continue as normal by rewriting the middle term, and factor by grouping to finish.

If you would like more help with this exercise, then please show your work and explain your reasoning, so that I can easily determine where to continue helping.

 

[stapel]

For an explanation of the "factors of (4)(-9) = -36 that add to -12" technique mentioned in the previous reply, try this online lesson. :wink:

Eliz.