(1+19/78×1)+(1+19/78×2)+(1+19/78×3)+……+(1+19/78×12)=?

问题描述:

(1+19/78×1)+(1+19/78×2)+(1+19/78×3)+……+(1+19/78×12)=?

=12x1+19/78x(1+12)x12/2=12+19=31