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?

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

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