Toán Lớp 5

Tính: 
(1-1/3) x (1-1/6) x (1-1/10) x (1-1/15)x ...x (1-1/780) = ???

Hướng dẫn:
(1-1/3) x (1-1/6) x (1-1/10) x (1-1/15)x ...x (1-1/780) 
= 2/3 x 5/6 x 9/10 x 14/15 x...x 779/780 
= 4/6 x 10/12 x 18/20 x 28/30 x...x 1558/1560 
= (4 x 10 x18 x 28 x...x 1558) / (6 x 12 x 20 x 30 x...x 1560) 
= (1 x 4 x 2 x 5 x 3 x 6 x 4 x 7 x ... x 37 x 40 x 38 x 41) / (2 x 3 x 3 x 4 x 4 x 5 x 5 x 6 x ... x 39 x 40) = (1x2x3x...x38 x 4x5x6x...x41) / (2x3x4x5x...x39 x 3x4x5x6x...x40) 
= 41 / (39 x 3) 
= 41 / 117