1-2/1*(1+2)-3/(1+2)*(1+2+3)-4/(1+2+3)*(1+2+3+4)-……-20/(1+2+3+……+19)*(1+2+3+……+20)=?
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![1-2/1*(1+2)-3/(1+2)*(1+2+3)-4/(1+2+3)*(1+2+3+4)-……-20/(1+2+3+……+19)*(1+2+3+……+20)=?](/uploads/image/z/370295-71-5.jpg?t=1-2%2F1%2A%281%2B2%29-3%2F%281%2B2%29%2A%281%2B2%2B3%29-4%2F%281%2B2%2B3%29%2A%281%2B2%2B3%2B4%29-%E2%80%A6%E2%80%A6-20%2F%281%2B2%2B3%2B%E2%80%A6%E2%80%A6%2B19%29%2A%281%2B2%2B3%2B%E2%80%A6%E2%80%A6%2B20%29%3D%3F)
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1-2/1*(1+2)-3/(1+2)*(1+2+3)-4/(1+2+3)*(1+2+3+4)-……-20/(1+2+3+……+19)*(1+2+3+……+20)=?
1-2/1*(1+2)-3/(1+2)*(1+2+3)-4/(1+2+3)*(1+2+3+4)-……-20/(1+2+3+……+19)*(1+2+3+……+20)=?
1-2/1*(1+2)-3/(1+2)*(1+2+3)-4/(1+2+3)*(1+2+3+4)-……-20/(1+2+3+……+19)*(1+2+3+……+20)=?
通项为-2/(n-1)+4/n+2/(n+1)
所得结果为1/210