SOLUTION: i need help with this factoring problem {{{ 7y^2+7y-18 }}} and also this one with the negative first kinda threw me off. {{{-5ab^2+10a^2b-15ab }}}

Algebra ->  Polynomials-and-rational-expressions -> SOLUTION: i need help with this factoring problem {{{ 7y^2+7y-18 }}} and also this one with the negative first kinda threw me off. {{{-5ab^2+10a^2b-15ab }}}      Log On


   



Question 168402: i need help with this factoring problem +7y%5E2%2B7y-18+
and also this one with the negative first kinda threw me off.
-5ab%5E2%2B10a%5E2b-15ab+

Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!
# 1



Looking at the expression 7y%5E2%2B7y-18, we can see that the first coefficient is 7, the second coefficient is 7, and the last term is -18.


Now multiply the first coefficient 7 by the last term -18 to get %287%29%28-18%29=-126.


Now the question is: what two whole numbers multiply to -126 (the previous product) and add to the second coefficient 7?


To find these two numbers, we need to list all of the factors of -126 (the previous product).


Factors of -126:
1,2,3,6,7,9,14,18,21,42,63,126
-1,-2,-3,-6,-7,-9,-14,-18,-21,-42,-63,-126


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


These factors pair up and multiply to -126.
1*(-126)
2*(-63)
3*(-42)
6*(-21)
7*(-18)
9*(-14)
(-1)*(126)
(-2)*(63)
(-3)*(42)
(-6)*(21)
(-7)*(18)
(-9)*(14)

Now let's add up each pair of factors to see if one pair adds to the middle coefficient 7:


First NumberSecond NumberSum
1-1261+(-126)=-125
2-632+(-63)=-61
3-423+(-42)=-39
6-216+(-21)=-15
7-187+(-18)=-11
9-149+(-14)=-5
-1126-1+126=125
-263-2+63=61
-342-3+42=39
-621-6+21=15
-718-7+18=11
-914-9+14=5



From the table, we can see that there are NO pairs of numbers which add to 7.


So 7y%5E2%2B7y-18 CANNOT be factored.







-5ab%5E2%2B10a%5E2b-15ab Start with the given expression


-5ab%28b-2a%2B3%29 Factor out the GCF -5ab


Since the inner term contains no other common terms or exponents, this is as far as we go.


So -5ab%5E2%2B10a%5E2b-15ab factors to -5ab%28b-2a%2B3%29