Sn=4n+n(n-1)/2乘6=3n^2+n怎麼化簡來的?

Sn=4n+n(n-1)/2乘6=3n^2+n怎麼化簡來的?

Sn=4n+n(n-1)/2x6
=4n+3n^2-3n
=3n^2+n