help please lost!

ab27834 · #1 $Old$ December 6th, 2008, 05:53 PM

I can multiply this easily-but the problem says to simplify it. I have the answer and NO idea how to get there. Can someone help to simplify:

(3x^2-2x^2-7)(8x^2-8x+7)

Please show steps-I'm lost! Thanks!

chella182 · #2 $Old$ December 6th, 2008, 06:00 PM

Is that first bracket meant to be $3x^2-2x^2-7$ or $3x^2-2x-7$ ?

ab27834 · #3 $Old$ December 6th, 2008, 06:10 PM

2x^2

chella182 · #4 $Old$ December 6th, 2008, 06:11 PM

So the first bracket would be $x^2-7$ then ?
Would you not just multiply out everything then collect the terms?

ab27834 · #5 $Old$ December 6th, 2008, 06:13 PM

first bracket:
3x^3-2x^2-7

second bracket:
8x^2-8x+7

problem says to simplify
thanks

ab27834 · #6 $Old$ December 6th, 2008, 06:16 PM

did that-but does not match the answer. Apparently, this can be simplified before solving. I don't get it, though.

chella182 · #7 $Old$ December 6th, 2008, 06:16 PM

Well I'd just expand and collect similar terms.
So expanding you'd get...

$24x^5-24x^4+21x^3-16x^4+16x^3-14x^2-56x^2+56x-49$

Then collecting similar terms you'd get...

$24x^5-40x^4+37x^3-70x^2+56x-49$

(Might want to check where I've collected the terms; it's very late and I did the adding & subtracting in my head haha!)

chella182 · #8 $Old$ December 6th, 2008, 06:17 PM

Quote:

Originally Posted by ab27834 $View Post$

did that-but does not match the answer. Apparently, this can be simplified before solving. I don't get it, though.

Solving what?

ab27834 · #9 $Old$ December 6th, 2008, 06:19 PM

That's what I got. THE BOOK SAYS the answer is: 3x^3-10x^2+8x+11

I'm COMPLETELY in the dark.

chella182 · #10 $Old$ December 6th, 2008, 06:23 PM

Well you're obviously going to get terms of order higher than $x^3$ if you're multiplying out those brackets

sorry I couldn't be of more help.

ab27834 · #11 $Old$ December 6th, 2008, 06:27 PM

Thanks anyway. I've been racking my brain for 2 days now. No one I've asked has gotten anything other than what I did, either.

chella182 · #12 $Old$ December 6th, 2008, 06:35 PM

Possibly a mis-print in the text book? I know when I was doing A-level Maths our solutions in the back of the book were prone to being less than accurate

ab27834 · #13 $Old$ December 6th, 2008, 06:52 PM

I'm hoping so. This is a study guide to a challenge exam-and I missed all but the first two. None of them makes sense. Hoping the whole &%%^^& thing is a misprint. I WAS pretty confident before I tried the sample test.