Question 177251: I'm suppose to factor:
34) 3x2 + 6x – 24
35) 4x2 + 4x –15
Answer by jim_thompson5910(35256)   (Show Source):
You can put this solution on YOUR website! # 34
 Start with the given expression
 Factor out the GCF 
Now let's focus on the inner expression 
------------------------------------------------------------
Looking at we can see that the first term is  and the last term is  where the coefficients are 1 and -8 respectively.
Now multiply the first coefficient 1 and the last coefficient -8 to get -8. Now what two numbers multiply to -8 and add to the middle coefficient 2? Let's list all of the factors of -8:
Factors of -8:
1,2,4,8
-1,-2,-4,-8 ...List the negative factors as well. This will allow us to find all possible combinations
These factors pair up and multiply to -8
(1)*(-8)
(2)*(-4)
(-1)*(8)
(-2)*(4)
note: remember, the product of a negative and a positive number is a negative number
Now which of these pairs add to 2? Lets make a table of all of the pairs of factors we multiplied and see which two numbers add to 2
| First Number | Second Number | Sum | | 1 | -8 | 1+(-8)=-7 | | 2 | -4 | 2+(-4)=-2 | | -1 | 8 | -1+8=7 | | -2 | 4 | -2+4=2 |
From this list we can see that -2 and 4 add up to 2 and multiply to -8
Now looking at the expression , replace  with  (notice adds up to . So it is equivalent to )
Now let's factor  by grouping:
 Group like terms
 Factor out the GCF of  out of the first group. Factor out the GCF of  out of the second group
 Since we have a common term of  , we can combine like terms
So factors to
So this also means that factors to (since is equivalent to )
------------------------------------------------------------
So our expression goes from and factors further to 
------------------
Answer:
So factors to
# 35
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 :
1,2,3,4,5,6,10,12,15,20,30,60
-1,-2,-3,-4,-5,-6,-10,-12,-15,-20,-30,-60
Note: list the negative of each factor. This will allow us to find all possible combinations.
These factors pair up and multiply to .
1*(-60)
2*(-30)
3*(-20)
4*(-15)
5*(-12)
6*(-10)
(-1)*(60)
(-2)*(30)
(-3)*(20)
(-4)*(15)
(-5)*(12)
(-6)*(10)
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 | -60 | 1+(-60)=-59 | | 2 | -30 | 2+(-30)=-28 | | 3 | -20 | 3+(-20)=-17 | | 4 | -15 | 4+(-15)=-11 | | 5 | -12 | 5+(-12)=-7 | | 6 | -10 | 6+(-10)=-4 | | -1 | 60 | -1+60=59 | | -2 | 30 | -2+30=28 | | -3 | 20 | -3+20=17 | | -4 | 15 | -4+15=11 | | -5 | 12 | -5+12=7 | | -6 | 10 | -6+10=4 |
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 
---------------------------------------------
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).
|
|
|
