b,tính 1/1x2+1/2x3+1/3x4+.....+1/99x100
c, 2/1x3+2/3x5+2/5x7+...+2/99x101
d, 1/1x3+1/3x5+1/5x7+...+1/99x101
Đăng ký qua Facebook hoặc Google: