là cái đồng hồ nha

1/1.2+1/2.3+1/3.4+1/4.5+1/5.6+1/6.7+1/7.8+1/8.9+1/9.10+1/10.11+1/11.12+1/12.13

= 1/1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10+1/10-1/11+1/1-1/12+1/12-1/13

= 1/1+0+1/13

= 14/13

Tính nhanh:

\(\frac{-2}{3}-\frac{1}{3}.\left(2.x-5\right)=\frac{3}{2}\)

Giúp mik vs ạ!