50-49+48-47+…+4-3+2-1=(50-49)+(48-47)+。。。。。+(4-3)+(2-1)=25
每两个数加括号,每一个都是1,共25组,结果25
50-49+48-47+…+4-3+2-1=(50-49)+(48-47)+…+(4-3)+(2-1)=1+1+……+1+1=1×50÷2=25
50-49+48-47+…+4-3+2-1=(50-49)+(48-47)+…+(4-3)+(2-1)=25