Tính nhanh: 1/7.5/9 + -5/9.2/-7 + 5/-9 .-1/7 + 5/9.3/-7

1 t ínhnhanh
a, 1/7 . 5/9 + -5/9 . 2/-7 + 5/-9 . -1/7 + 5/9 . 3/-7
b, 4/20 + 16/42 + 6/15 + -3/5 + 2/21 + -10/21 +-3/20
c, 4/2.9 + 4/9.16+ 4/16.23 +......+ 4/142.149
d, 3/5.13+ 3/13.21 + 3/21.29 +......+ 3/166.174