Thread: Need a little freshened up

In a question like: C(n,2)=45

n(n-1)=90
n2-n-90=0
(n-10)(n+9)=0 <- how do you know which to make the positive and which to make the negative?

solve n+9=0 and n-10=0. n=10. 10 is positive. Just solve both after setting both to 0.