Question 551510: Megan factored the expression 28x^2 + x - 15 as (4x-3)(7x+5). But when Jacob applied the FOIL principle and multiplied out (4x-3)(7x+5), he got 28x^2 - x - 15; thus, Megan’s solution does not appear to check. Why is that? Please help Megan to understand this better. Explain your reasoning and correctly factor the original expression, if possible. If the expression is prime, so state.
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) = -420
2*(-210) = -420
3*(-140) = -420
4*(-105) = -420
5*(-84) = -420
6*(-70) = -420
7*(-60) = -420
10*(-42) = -420
12*(-35) = -420
14*(-30) = -420
15*(-28) = -420
20*(-21) = -420
(-1)*(420) = -420
(-2)*(210) = -420
(-3)*(140) = -420
(-4)*(105) = -420
(-5)*(84) = -420
(-6)*(70) = -420
(-7)*(60) = -420
(-10)*(42) = -420
(-12)*(35) = -420
(-14)*(30) = -420
(-15)*(28) = -420
(-20)*(21) = -420
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)=-419 | | 2 | -210 | 2+(-210)=-208 | | 3 | -140 | 3+(-140)=-137 | | 4 | -105 | 4+(-105)=-101 | | 5 | -84 | 5+(-84)=-79 | | 6 | -70 | 6+(-70)=-64 | | 7 | -60 | 7+(-60)=-53 | | 10 | -42 | 10+(-42)=-32 | | 12 | -35 | 12+(-35)=-23 | | 14 | -30 | 14+(-30)=-16 | | 15 | -28 | 15+(-28)=-13 | | 20 | -21 | 20+(-21)=-1 | | -1 | 420 | -1+420=419 | | -2 | 210 | -2+210=208 | | -3 | 140 | -3+140=137 | | -4 | 105 | -4+105=101 | | -5 | 84 | -5+84=79 | | -6 | 70 | -6+70=64 | | -7 | 60 | -7+60=53 | | -10 | 42 | -10+42=32 | | -12 | 35 | -12+35=23 | | -14 | 30 | -14+30=16 | | -15 | 28 | -15+28=13 | | -20 | 21 | -20+21=1 |
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 .
In other words,  .
Note: you can check the answer by expanding to get or by graphing the original expression and the answer (the two graphs should be identical).
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