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College Algebra Factoring - Snickerdoodle - 09-12-2012
Hi all,
I'm having trouble with this flashcard. I thought there should be 2 factored out, and it says so in the explaination, but it's not in the answer. Any help is much appreciated!
Question 1 of 1
Factor:
18x2-14x-4
Your Answer: 2(9x+2)(x-1)
Explanation
Correct Answer: (9x+1)(2x-1)
Given:
18x2-14x-4
First notice that 2 can be factored out from every term in the polynomial above.
This gives us: 2(9x2 - 7x - 2)
The inside can be factored further:
9x2 - 7x - 2 -- This is of the form:
Ax2+Bx+C
When you see this form,
Begin to factor by drawing two parentheses
( ___ _ ___ )( ___ _ ___ )
(Step 1) Determine the sign (+ or -) that goes in the middle of each parentheses.
( __ ? __ )( __ ? __ )
How?:
Look at the last sign:
9x2 - 7x - 2
if the last sign is (-), as it is here, then the signs alternate: ( __ - __ )( __ + __ )
NOTICE:If it is (+), the two signs will be the same.
Either ( __ + __ )( __ + __ ) or ( __ - __ )( __ - __ ).
(Step 2) Next determine the first and last numbers for each parentheses.
( ? - ? )( ? + ? )
How?
List the factors of the first and last terms:
9x2 - 7x - 2
Since the last sign, as we determined above, is (-), the signs are different in either parentheses
and therefore the factors in the above table must SUBTRACT to the middle term of 14x.
And since 9x - 2x = 7x, we have picked the correct factors of the first and last terms.
The factors of the first and last terms are arranged in the parentheses in this way every time:
( # +/- # )( # +/- # )
Now look to see if there is a (+) or (-) sign in the middle:
9x2 - 7x - 2
The middle term is (-), and since the A term is 18, we need to perform a "check"
before we know where to place the (-) sign.
If we place the (-) sign in the first parentheses:
(9x-1)(2x+1)
Then Check:
(1) Multiply the inner terms: -1 * 2x = -2x
(2) Multiply the outer terms: 9x * (+1) = 9x
Then add -2x+9x = +7x
* Therefore this is wrong; our middle term is NOT (+)
If we place the (-) sign in the second parentheses:
(9x+1)(2x-1)
Then Check:
(1)Multiply the inner terms:+1 * (2x) = +2x
(2) Multiply the outer terms:9x * (-12) = -9x
Then add +2x-9x = -7x
* This IS our middle term, therefore this is correct!
Therefore our solution is:
(9x+1)(2x-1), which can also be written (2x-1)(9x+1).
College Algebra Factoring - 29palms - 09-13-2012
Forgive me, but I was just thinking this. What better opportunity it is for me to CUT AND RUN! See you later and good luck!
College Algebra Factoring - Snickerdoodle - 09-13-2012
Run, run, run away, I will find some other way. Anyone brave enough to slay this giant problem for me?
College Algebra Factoring - NAP - 09-13-2012
I haven't read the logic for the answer, but when I check your answer, it looks correct. It multiplies back to 18x^2-14x-4. Their answer multiplies back to 18x^2-7x-1. I think you understand how to do this one.
College Algebra Factoring - Publius - 09-13-2012
I looked at problem, and you got the right answer. I even double checked it with the MathWay Tool found on Free-CLEP-Prep.com and they also came up with 2(9x+2)(x-1). (MathWay Tool - Free-Clep-Prep.com)
Pretty sure they just goofed up. Esp. when it's in the explanation.
College Algebra Factoring - Snickerdoodle - 09-13-2012
Thanks a lot guys! I just need some confirmation that I'm not seeing things like I'm crazy.
Whew! And thanks Publius for the link. It looks very useful.