25+26-12-13+27+28-14-15+29+30-16-17怎么巧算

问题描述:

25+26-12-13+27+28-14-15+29+30-16-17怎么巧算

两个等差数列
(25+26+……+30)—(12+13+……+17)

很简单 25+26+27+28+29+30-12-13-14-15-16-17= (25-12)+ (26-13)+ (27-14)+ (28-15)+ (29-16)+ (30-17)=5x13=65