Question 199600: I'm not to sure if this is the right section for my question but here goes:
I'm working on factorizing quadratic trinomials and here is a question I need help with factorizing this expression.
1) 20x^2-44x+21
Found 2 solutions by vleith, jim_thompson5910:
Answer by vleith(2983)   (Show Source):
You can put this solution on YOUR website! To factor the quadratic, you need to look at the factors for the coefficients of of leading term (20) and the trailing term (21). Then decide which pairs of factors result in a difference of -44
The factor pairs for 20 are
1,20
2,10
4,5
The factor pairs for 21 are
1,21
3,7
So now you use those pairs of numbers and try to find sums/difference of products that get you to -44
So let's try 2,10 and 1,21
Does 2*1 + 10*21 = -44 no
Does 2*21 + 1*10 = - 44 No
OK, let's try something esle
Try 2,10 3,7
Does 2*7 + 10*3 = -44 No, but it does equal 44. We are getting closer. All we need to do now is make the signs correct
Here is a good URL for checking answers on this type of thing
http://72.3.253.76:8080/webMathematica3/quickmath/page.jsp?s1=algebra&s2=factor&s3=basic
Answer by jim_thompson5910(35256) (Show Source):
You can put this solution on YOUR website!
Looking at the expression , we can see that the first coefficient is  , the second coefficient is  , and the last term is  .
Now multiply the first coefficient by the last term to get  .
Now the question is: what two whole numbers multiply to  (the previous product) and add to the second coefficient ?
To find these two numbers, we need to list all of the factors of (the previous product).
Factors of :
1,2,3,4,5,6,7,10,12,14,15,20,21,28,30,35,42,60,70,84,105,140,210,420
-1,-2,-3,-4,-5,-6,-7,-10,-12,-14,-15,-20,-21,-28,-30,-35,-42,-60,-70,-84,-105,-140,-210,-420
Note: list the negative of each factor. This will allow us to find all possible combinations.
These factors pair up and multiply to .
1*420
2*210
3*140
4*105
5*84
6*70
7*60
10*42
12*35
14*30
15*28
20*21
(-1)*(-420)
(-2)*(-210)
(-3)*(-140)
(-4)*(-105)
(-5)*(-84)
(-6)*(-70)
(-7)*(-60)
(-10)*(-42)
(-12)*(-35)
(-14)*(-30)
(-15)*(-28)
(-20)*(-21)
Now let's add up each pair of factors to see if one pair adds to the middle coefficient :
| First Number | Second Number | Sum | | 1 | 420 | 1+420=421 | | 2 | 210 | 2+210=212 | | 3 | 140 | 3+140=143 | | 4 | 105 | 4+105=109 | | 5 | 84 | 5+84=89 | | 6 | 70 | 6+70=76 | | 7 | 60 | 7+60=67 | | 10 | 42 | 10+42=52 | | 12 | 35 | 12+35=47 | | 14 | 30 | 14+30=44 | | 15 | 28 | 15+28=43 | | 20 | 21 | 20+21=41 | | -1 | -420 | -1+(-420)=-421 | | -2 | -210 | -2+(-210)=-212 | | -3 | -140 | -3+(-140)=-143 | | -4 | -105 | -4+(-105)=-109 | | -5 | -84 | -5+(-84)=-89 | | -6 | -70 | -6+(-70)=-76 | | -7 | -60 | -7+(-60)=-67 | | -10 | -42 | -10+(-42)=-52 | | -12 | -35 | -12+(-35)=-47 | | -14 | -30 | -14+(-30)=-44 | | -15 | -28 | -15+(-28)=-43 | | -20 | -21 | -20+(-21)=-41 |
From the table, we can see that the two numbers  and  add to (the middle coefficient).
So the two numbers and both multiply to and add to
Now replace the middle term  with  . Remember, and add to . So this shows us that  .
 Replace the second term with .
 Group the terms into two pairs.
 Factor out the GCF  from the first group.
 Factor out  from the second group. The goal of this step is to make the terms in the second parenthesis equal to the terms in the first parenthesis.
 Combine like terms. Or factor out the common term 
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Answer:
So factors to .
Note: you can check the answer by FOILing to get or by graphing the original expression and the answer (the two graphs should be identical).
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