Tính

Tính :
​A = ​$\frac{1}{1\mathrm{x}2}+\frac{1}{2x3}+\frac{1}{3x4}+.......+\frac{1}{98x99}+\frac{1}{99x100}$

B = $\frac{1}{1\mathrm{x}3}+\frac{1}{3x5}+\frac{1}{5x7}+.........+\frac{1}{97x99}+\frac{1}{99x101}$