Tính nhanh
1//21 + 1/28 + 1/36 + ...... + 2/x(x+1) = 2/9
1/21+1/28+1/36+...+2/x(x+1)=2/9
1/21 + 1/28 +1/36 +... + 2/x(x+1) = 2/9
1/21+1/28+1/36+...+2/x(x+1)=2/9
Tìm x: 1/21+1/28+1/36+...+2/x.(x+1)=2/9
b. 1/21+1/28+1/36+...+2/x.(x+1)=2/9
b. 1/21+1/28+1/36+...+2/x.(x+1)=2/9
tìm x: 1/21+1/28+1/36+...+1/x.(x+1)=2/9
