(2-3*5^-1)+(4-3*5^-2）+...+(2n-3*5^-n)
=2+4+6+8+……+2n-3（5^-1+5^-2+5^-3+……+5^-n)
=n^2+n-3*(1/5)*(1-(1/5)^n)/(1-1/5)
=n^2+n-3/4+3/4*(1/5)^n