n*n-3n=108n*n-3n-108=0(n-12)(n+9)=0所以n=12或-9
(n·n-3n)/2=54 (n^2-3n)=108n^2-3n-108=0(n-12)(n+9)=0n=12 n=-9
(n·n-3n)/2=54 (n^2-3n)=108 n^2-3n-108=0 (n-12)(n+9)=0 n=12或 -9