SOLUTION: Factorise 3x^2 + 15x-42

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Question 807347: Factorise 3x^2 + 15x-42
Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!

3x%5E2%2B15x-42 Start with the given expression.


3%28x%5E2%2B5x-14%29 Factor out the GCF 3.


Now let's try to factor the inner expression x%5E2%2B5x-14


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Looking at the expression x%5E2%2B5x-14, we can see that the first coefficient is 1, the second coefficient is 5, and the last term is -14.


Now multiply the first coefficient 1 by the last term -14 to get %281%29%28-14%29=-14.


Now the question is: what two whole numbers multiply to -14 (the previous product) and add to the second coefficient 5?


To find these two numbers, we need to list all of the factors of -14 (the previous product).


Factors of -14:
1,2,7,14
-1,-2,-7,-14


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


These factors pair up and multiply to -14.
1*(-14) = -14
2*(-7) = -14
(-1)*(14) = -14
(-2)*(7) = -14

Now let's add up each pair of factors to see if one pair adds to the middle coefficient 5:


First NumberSecond NumberSum
1-141+(-14)=-13
2-72+(-7)=-5
-114-1+14=13
-27-2+7=5



From the table, we can see that the two numbers -2 and 7 add to 5 (the middle coefficient).


So the two numbers -2 and 7 both multiply to -14 and add to 5


Now replace the middle term 5x with -2x%2B7x. Remember, -2 and 7 add to 5. So this shows us that -2x%2B7x=5x.


x%5E2%2Bhighlight%28-2x%2B7x%29-14 Replace the second term 5x with -2x%2B7x.


%28x%5E2-2x%29%2B%287x-14%29 Group the terms into two pairs.


x%28x-2%29%2B%287x-14%29 Factor out the GCF x from the first group.


x%28x-2%29%2B7%28x-2%29 Factor out 7 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%2B7%29%28x-2%29 Combine like terms. Or factor out the common term x-2


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So 3%28x%5E2%2B5x-14%29 then factors further to 3%28x%2B7%29%28x-2%29


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Answer:


So 3x%5E2%2B15x-42 completely factors to 3%28x%2B7%29%28x-2%29.


In other words, 3x%5E2%2B15x-42=3%28x%2B7%29%28x-2%29.


Note: you can check the answer by expanding 3%28x%2B7%29%28x-2%29 to get 3x%5E2%2B15x-42 or by graphing the original expression and the answer (the two graphs should be identical).