Bài 1 :
a, \(\frac{1}{2\times3}+\frac{1}{3\times4}+\frac{1}{4\times5}+...+\frac{1}{101\times102}\)
b, \(\frac{5}{5\times10}+\frac{5}{10\times15}+...+\frac{5}{115\times120}\)
c, \(\frac{2}{5\times7}+\frac{2}{7\times9}+\frac{2}{9\times11}+...+\frac{2}{997\times999}\)