Tìm x,biết
a)(x+1)+(x+2)+........+(x+100)=5750
b)\(\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+........+\frac{1}{x\cdot\left(x+1\right)}=\frac{2015}{2016}\)
c)\(\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+..........+\frac{2}{x\cdot\left(x+1\right)}=\frac{2}{9}\)