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#1 2008-04-14 19:16:24
- soha
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- Registered: 2006-07-07
- Posts: 2,530
co efficients
Find the co efficients of x and x² in the following products:-
(3x-1)(3x-5)(3x+4)
"Let us realize that: the privilege to work is a gift, the power to work is a blessing, the love of work is success!" 
- David O. McKay
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#2 2008-04-15 04:08:35
- Dragonshade
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- Registered: 2008-01-16
- Posts: 147
Re: co efficients
For 3x could only choose (-5 ,4). -1 could choose (3x,4) and (-5,3x)
3x*(-5)*4+(-1)*3x*4+(-1)*(-5)*3x = -60x-12x+15x=-57x
I'll leave the second one to you
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#3 2008-04-15 04:51:07
- LuisRodg
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- Registered: 2007-10-23
- Posts: 322
Re: co efficients
Expanding it is the fastest way in my opinion
So coefficient of x^3 is 27, coefficient of x^2 is -18 and coefficient of x is -57.
Last edited by LuisRodg (2008-04-15 04:53:10)
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#4 2008-04-15 06:01:16
- Ricky
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- Registered: 2005-12-04
- Posts: 3,791
Re: co efficients
(3x-1)(3x-5)(3x+4)
Going for the coefficient of x^2:
3x*3x*4 = 36x^2
3x * -5 * 3x = -45x^2
-1 * 3x * 3x = -9x^2
x^2: -18
This won't have much of an advantage over expanding the entire thing for small multiplications, but as you get larger this method is probably prefered.
"In the real world, this would be a problem. But in mathematics, we can just define a place where this problem doesn't exist. So we'll go ahead and do that now..."
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#5 2008-04-15 18:38:35
- soha
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- Registered: 2006-07-07
- Posts: 2,530
Re: co efficients
thanks
"Let us realize that: the privilege to work is a gift, the power to work is a blessing, the love of work is success!"
- David O. McKay
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#6 2008-04-15 21:59:43
- John E. Franklin
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- Registered: 2005-08-29
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Re: co efficients
It is interesting to compare this type of problem
to finding the volume of a cuboid or rectangular "cube".
If each side is broken into 2 parts, then
you get:
volume = (a+b)(c+d)(e+f)
Note that this is different than
finding the area of the cuboid:
surface area = 2(a+b)(c+d) + 2(a+b)(e+f) + 2(c+d)(e+f)
The volume again broken up is:
volume = ace + ade + acf + adf + bce + bde + bcf + bdf
igloo myrtilles fourmis
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