Home sick from school, need help with algebra (trying not to fall behind)

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DustyWusty

Banned
Nov 18, 2003
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Could someone walk me through how to do this problem? I can usually figure this stuff out, but its just been confusing me lately (dont laugh i know it must be easy for you guys lol)

2m^2 - 11m - 21

I thought you would take the -21 and find two factors that = -11 but i guess not, as either I cant find any or its 1 or something, I confuse myself, im only in algebra 1 and I didnt have math last year so go easy guys :)
 
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GtPrOjEcTX

Lifer
Jul 3, 2001
10,784
6
81
you don't have many choices for factors...just 1, 3, 7, and 21. Well, 1 and 21 aren't going to get you 11. so you use 3 and 7, but one of those has to be multipled by 2 (because 2m*m = 2m^2). So 2x3=6... can't do anything with 6 and 7 to get 11, so 2*7=14. 14-3=11 so we have a winner.

so now you just have to find out were to put the negatives.
 
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Reel

Diamond Member
Jul 14, 2001
4,484
0
76
You know since m^2 is the highest ordered variable and the last term is negative that you will have something in the form of (m + something) * (m - something). Since m^2 has a coefficient of 2, you need to have one of those m's in the answer have a 2 coefficient as well. Then you just take the -21 and figure out the factors of it: 3 7, 1 21. Now try out those options in place of the "something" listed above and see what works.

Edit: slightly different explanation of above but same end result. Out of curiosity, what is your background Gt (major, job, school, whatever)?
 
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ThaGrandCow

Diamond Member
Dec 27, 2001
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EDIT: That's what I get for staying up all night. I really should go to bed at some point
 
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GtPrOjEcTX

Lifer
Jul 3, 2001
10,784
6
81
Originally posted by: ReelC00L
You know since m^2 is the highest ordered variable and the last term is negative that you will have something in the form of (m + something) * (m - something). Since m^2 has a coefficient of 2, you need to have one of those m's in the answer have a 2 coefficient as well. Then you just take the -21 and figure out the factors of it: 3 7, 1 21. Now try out those options in place of the "something" listed above and see what works.

Edit: slightly different explanation of above but same end result. Out of curiosity, what is your background Gt (major, job, school, whatever)?
Click to expand...
Just finished my BS. Major in Comp. Science, with Minor in Math.
 
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DustyWusty

Banned
Nov 18, 2003
55
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Im a junior in highschool, haven't decided on college or anything yet, no job yet (been selling stuff and running things around for people since I have a pickup) EDIT just realized you guys didnt ask me that lol

Im doing what you guys are saying but im getting

2m^2 - 11m + 21

instead of a -21

i must be doing something wrong

thanks for the help guys

EDIT!!
Im so st00pid
a - + a + is -

cryptic huh lol
 
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Ogg

Diamond Member
Sep 5, 2003
4,829
1
0
Originally posted by: DustyWusty
ran into a snag

7r^2 - 23r - 20
(7r-4) (r+5)
but its not right when I check it

anyone?
Click to expand...

switch em around

(+5) ( -4)
 
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Ogg

Diamond Member
Sep 5, 2003
4,829
1
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Originally posted by: SpazzyChicken
Originally posted by: DustyWusty
ran into a snag

7r^2 - 23r - 20
(7r-4) (r+5)
but its not right when I check it

anyone?
Click to expand...

Switch the -4 and +5

Should be (7r + 5) (r - 4)
Click to expand...

damn you chicken..........beat me by seconds:D
 
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SpazzyChicken

Senior member
Feb 8, 2002
843
1
0
Originally posted by: DustyWusty
..silly me, multiplying backwards or something

sorry I suck at math so bad lol
Click to expand...


No problem man. Don't beat yourself up about it, you don't suck at math. I think that it is great that you are working it even though you are home sick. Stick with it and work hard at it (like you currently are). Ask questions!! I'll be happy to help you with any of your problems. Just don't ask me to do your homework for you!

<------- recently graduated B.S. in Mathematics :D
 
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SpazzyChicken

Senior member
Feb 8, 2002
843
1
0
Originally posted by: Joker81
isn't it x = -b +- squareroot(b^2-4a*c) / 2a
Click to expand...

that would be true if we were solving for x, but here we are just factoring. And we also can't assume that the 2m^2 -11m -21 = 0.
 
M

Muzzan

Member
Apr 15, 2003
169
0
0
Well, you could set 2m^2 - 11m - 21 equal to 0, find its roots (with, say, the quadratic formula) and from that it would follow that 2m^2 - 11m - 21 = 2(m - a)(m - b), where a and b are the roots.
 
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