7+12+17+22+27+.+5n+2=

7+12+17+22+27+.+5n+2=

원 식=1*5+2+2*5+2+...+n*5+2
=5*(1+2+3+...+n)+2*n
=5*(1+n)*n/2+2n
=2.5n^2+2.5n+2n
=2.5n^2(n 제곱)+4.5n