Cau 2

F= 1/ 11 x 16 + 1/16x21 + 1/21 x 26 +.....+ 1/56x61 + 1/ 61 x 66
F = 1/5.(5/11.16 + 5/16.21 + 5/21.26 + ... + 5/56.61 + 1/61.66)
F = 1/5.(1/11 - 1/16 + 1/16 - 1/21 + 1/21 - 1/26 + ... + 1/56 - 1/61 + 1/61 - 1/66)
F = 1/5.(1/11 - 1/66)
F = 1/5 . 5/66
F = 1/66

5F = 1/11 - 1/16 + 1/16 - 1/21 + 1/21 - 1/26 +.........+ 1/56 - 1/61 +1/61 - 1/66
5F = 1/11 - 1/66
5F = 5/66
F= 1/66

F = 1/11.16 + 1/16.21 + 1/21.26 +...+ 1/56.61 + 1/61.66
5F = 5/11.16 + 5/16.21 + 5/21.26 +...+ 5/56.61 + 5/61.66
5F = (16 - 11)/11.16 + (21 - 16)/16.21 + (26 - 21)/21.26 +...+ (61 - 56/56.61 + (66 - 61)/61.66
5F = 1/11 - 1/16 + 1/16 - 1/21 + 1/21 - 1/26 +...+ 1/56 - 1/61 + 1/61 - 1/66
5F = 1/11 - 1/66
5F = 5/66
F = 1/66

B= 1/3 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729 B x 3 = 3 x (1/3 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729)
= 1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243
B x 3 -B = 1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243 - (1/3 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729)
= 1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243 - 1/3 - 1/9 - 1/27 - 1/81 - 1/243 - 1/729
= 1 - 1/729
B x 2 = 728/729
B = 364/729

F = 1/11.16 + 1/16.21 + 1/21.26 +...+ 1/56.61 + 1/61.66
5F = 5/11.16 + 5/16.21 + 5/21.26 +...+ 5/56.61 + 5/61.66
5F = (16 - 11)/11.16 + (21 - 16)/16.21 + (26 - 21)/21.26 +...+ (61 - 56/56.61 + (66 - 61)/61.66
5F = 1/11 - 1/16 + 1/16 - 1/21 + 1/21 - 1/26 +...+ 1/56 - 1/61 + 1/61 - 1/66
5F = 1/11 - 1/66
5F = 5/66
F = 1/66