SOLUTION: Okay so I'm doing the factoring of trinomials when you have a coeffcient and your x is negative. The problem is -x^2+x+20 and you have to factor it out and find the roots, and I'm

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Question 857602: Okay so I'm doing the factoring of trinomials when you have a coeffcient and your x is negative.
The problem is -x^2+x+20 and you have to factor it out and find the roots, and I'm trying to factor it out so that when I distribute, I get the original equation, but I'm not getting it. As far as I know the roots are -4, and 5 but I'm not understanding why those are the roots. Can you help with the factoring and the roots?
Answer by richwmiller(17219)   (Show Source): You can put this solution on YOUR website!
You can also factor into ((-x-4)*(x-5)) or ((x+4)*(5-x))
Solved by pluggable solver: Factoring using the AC method (Factor by Grouping)


Start with the given expression.



Factor out the GCF .



Now let's try to factor the inner expression



---------------------------------------------------------------



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 :

1,2,4,5,10,20

-1,-2,-4,-5,-10,-20



Note: list the negative of each factor. This will allow us to find all possible combinations.



These factors pair up and multiply to .

1*(-20) = -20
2*(-10) = -20
4*(-5) = -20
(-1)*(20) = -20
(-2)*(10) = -20
(-4)*(5) = -20


Now let's add up each pair of factors to see if one pair adds to the middle coefficient :



First NumberSecond NumberSum
1-201+(-20)=-19
2-102+(-10)=-8
4-54+(-5)=-1
-120-1+20=19
-210-2+10=8
-45-4+5=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



--------------------------------------------------



So then factors further to



===============================================================



Answer:



So completely 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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