Tính nhanh: A = 1/9.15/22.-9/25

B1: tính nhanh
A= 1/9 . 15/22 . -9/25
B= -3/5 . -4/7 .- 5/6 . -7/8
B2:tính
(1-1/2).(1-1/3).(1-1/4)...(1-1/2010)
B3:
a) 4/9 . (5/7 + 2/5) - 4/9 . (-2/7 + 7/5)
b) 1/3 . 5/11 +4/13. 6/11+4/13.6/11-4/13
B4:
a) 1/-5.5/7 b)-2/7.7/10 c)(-5) . 7/20 d) -1/3.(-7)