Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

Need help solving an equation by factoring.

PROBLEM: $3v^2-10v-12 = -28v + 36$

This is my solution, but it seems a little too much for school:

$3v^2+18v-48 = 0$

$3(v^2+18v-48) = 0$

$3((-1v + -8)(-1v + 2)) = 0$

$-1v + -8 = 0$

$v = -8$

$-1v + 2 = 0$

$v = 2$

share|improve this question
    
I don't know if it's too much for school, but it's wrong for math. Careful when you factor out that $3$! –  Gerry Myerson Jan 29 '13 at 3:19
    
Your second equation is incorrect. Then the factorization is mysteriously correct. –  1015 Jan 29 '13 at 3:26
add comment

1 Answer

up vote 2 down vote accepted

Your algebra went astray: from $3v^2-10v-12=-28v+36$ you should get

$$3v^2+18v-48=0\;.$$

Then you can factor out the $3$ to get $3(v^2+6v-16)=0$, so $v^2+6v-16=0$, and this then factors nicely to give you $(v+8)(v-2)=0$.

You actually made compensating errors, since $3(v^2+18v-48)$ does not factor as

$$3\big((−1v+−8)(−1v+2)\big)\;;$$

your $$3\big((−1v+−8)(−1v+2)\big)=0$$ is actually equivalent to

$$3(v+8)(v-2)=0\;,$$

which is essentially what I got.

share|improve this answer
    
OP got the right factors, so the misfactoring of the $3$ looks like a transcription error. –  Gerry Myerson Jan 29 '13 at 3:21
    
My answer is probably wrong because I've been using online calculators to check my work. I guess I can't rely on them. –  12japerk Jan 29 '13 at 3:23
    
@Gerry: I realized that just after I posted the original version; I can’t tell whether it’s a transcription error or just sloppy notation $-$ I’ve seen plenty of both! –  Brian M. Scott Jan 29 '13 at 3:25
    
Thanks for your post Brian, It helped a lot. –  12japerk Jan 29 '13 at 3:26
    
@12japerk: You’re welcome. –  Brian M. Scott Jan 29 '13 at 3:26
show 4 more comments

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.