等差数列的求和公式?

xiuchang

who can send me? thank you very much. I will take the test within 2 days la..

rainating

Sn=(a1+an)*n/2

BruceChiang

The sum formula of equal-difference series seems like (a1+an)n/2 that is the same as the area formula of Trapezoid.

FYR~