Tính nhanh:
a)\(\frac{131313}{151515}+ \frac{131313}{353535}+\frac{131313}{636363}+\frac{131313}{999999}\)
b)\(\frac{1}{1\times2}+\frac{1}{2\times3}+\frac{1}{3\times4}+...+\frac{1}{2017\times2018}\)
c)\(\frac{1}{3\times5}+\frac{1}{4\times7}+\frac{1}{7\times9}+...\frac{1}{2015\times2017}+\frac{1}{2017\times2018}\)