SOLUTION: can you factor x squared - x - 20 ? Thank you. Amanda

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Question 7696: can you factor x squared - x - 20 ? Thank you. Amanda
Answer by drglass(89) About Me  (Show Source):
You can put this solution on YOUR website!
When we factor an equation we end up with something like (x + a)(x + b). If we complete the multiplication, we get x%5E2+%2B+%28a+%2B+b%29x+%2B+ab. To factor your specific equation, we have to find the values of a and b that give us x%5E2+-x+-+20. In other words %28a+%2B+b%29+=+-1 and ab+=+-20.
There are a few ways to get ab+=+-20 but also need to find values for a and b that gives a+%2B+b+=+-1 
  a      b     a + b
 20     -1       19
-20      1      -19
  1    -20      -19
 -1     20       19
 10     -2        8
-10      2       -8
  2    -10       -8
 -2     10        8
  5     -4        1
 -5      4       -1 this works
  4     -5       -1 this works
 -4      5        1

We can see that a = 5 and b = -4, or a = -4 and b = 5 works, so which do we choose? Well, it does not really matter because they both give the same answer:
(x - 5)(x + 4) = x%5E2+-+x+-20 and
(x + 4)(x - 5) = x%5E2+-+x+-20