SOLUTION: x^2-x-20 I need to know how to find the factor of this polynominal?

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Question 93008: x^2-x-20
I need to know how to find the factor of this polynominal?
Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!
Solved by pluggable solver: Factoring using the AC method (Factor by Grouping)


Looking at the expression x%5E2-x-20, we can see that the first coefficient is 1, the second coefficient is -1, and the last term is -20.



Now multiply the first coefficient 1 by the last term -20 to get %281%29%28-20%29=-20.



Now the question is: what two whole numbers multiply to -20 (the previous product) and add to the second coefficient -1?



To find these two numbers, we need to list all of the factors of -20 (the previous product).



Factors of -20:

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 -20.

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 -1:



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 4 and -5 add to -1 (the middle coefficient).



So the two numbers 4 and -5 both multiply to -20 and add to -1



Now replace the middle term -1x with 4x-5x. Remember, 4 and -5 add to -1. So this shows us that 4x-5x=-1x.



x%5E2%2Bhighlight%284x-5x%29-20 Replace the second term -1x with 4x-5x.



%28x%5E2%2B4x%29%2B%28-5x-20%29 Group the terms into two pairs.



x%28x%2B4%29%2B%28-5x-20%29 Factor out the GCF x from the first group.



x%28x%2B4%29-5%28x%2B4%29 Factor out 5 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.



%28x-5%29%28x%2B4%29 Combine like terms. Or factor out the common term x%2B4



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Answer:



So x%5E2-x-20 factors to %28x-5%29%28x%2B4%29.



In other words, x%5E2-x-20=%28x-5%29%28x%2B4%29.



Note: you can check the answer by expanding %28x-5%29%28x%2B4%29 to get x%5E2-x-20 or by graphing the original expression and the answer (the two graphs should be identical).