Tính nhanh

1. Tính nhanh:
b, 3/1x4 + 3/4x7 + 3/7x10 + .....+ 3/2020x2023
c, 5/14 + 5/84 + 5/204 + 5/374
d, 2/15 + 2/35 + 2/63 +.....+ 2/2021x2023
2. Tính nhanh:
a, 3/2x5 + 3/5x8 + 3/8x11 +.....+ 3/200x203
b, 2/5x7 + 2/7x9 + 2/9x11 +....+ 2/1999x2001
c, 5/6 + 5/66 + 5/176 + 5/336.