Question

Đã trả lời bởi chuyên gia

Answer 1

Ta có:

$\frac{1}{2} + \frac{1}{6} + \frac{1}{12} + \frac{1}{20} = \frac{2 - 1}{2} + \frac{3 - 2}{6} + \frac{4 - 3}{12} + \frac{5 - 4}{20} = \frac{2}{2} - \frac{1}{2} + \frac{3}{6} - \frac{2}{6} + \frac{4}{12} - \frac{3}{12} + \frac{5}{20} - \frac{4}{20} = 1 - \frac{1}{2} + \frac{1}{2} - \frac{1}{3} + \frac{1}{3} - \frac{1}{4} + \frac{1}{4} - \frac{1}{5} = 1 - (\frac{1}{2} - \frac{1}{2}) - (\frac{1}{3} - \frac{1}{3}) - (\frac{1}{4} - \frac{1}{4}) - \frac{1}{5} = 1 - 0 - 0 - 0 - \frac{1}{5} = 1 - \frac{1}{5} = \frac{5}{5} - \frac{1}{5} = \frac{4}{5}$

...Xem thêm

Answer 2

Ta có:

$\frac{1}{2} + \frac{1}{6} + \frac{1}{12} + \frac{1}{20} = \frac{2 - 1}{2} + \frac{3 - 2}{6} + \frac{4 - 3}{12} + \frac{5 - 4}{20} = \frac{2}{2} - \frac{1}{2} + \frac{3}{6} - \frac{2}{6} + \frac{4}{12} - \frac{3}{12} + \frac{5}{20} - \frac{4}{20} = 1 - \frac{1}{2} + \frac{1}{2} - \frac{1}{3} + \frac{1}{3} - \frac{1}{4} + \frac{1}{4} - \frac{1}{5} = 1 - (\frac{1}{2} - \frac{1}{2}) - (\frac{1}{3} - \frac{1}{3}) - (\frac{1}{4} - \frac{1}{4}) - \frac{1}{5} = 1 - 0 - 0 - 0 - \frac{1}{5} = 1 - \frac{1}{5} = \frac{5}{5} - \frac{1}{5} = \frac{4}{5}$

Answer 3

1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90
= (2-1)/2 + (3-2)/6 + (4-3)/12 + (5-4)/20 + (6-5)/30 + (7-6)/42 + (8-7)/56 + (9-8)/72 + (10-9)/90
=1 -1/2 + 1/2 - 1/3 +1/3 -1/4 + 1/4 -1/5 + 1/5 - 1/6 + 1/6 -1/7 + 1/7 -1/8 +1/8 -1/9 + 1/9 -1/10
= 1 - 1/10
= 9/10

Answer 4

1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90
= (2-1)/2 + (3-2)/6 + (4-3)/12 + (5-4)/20 + (6-5)/30 + (7-6)/42 + (8-7)/56 + (9-8)/72 + (10-9)/90
=1 -1/2 + 1/2 - 1/3 +1/3 -1/4 + 1/4 -1/5 + 1/5 - 1/6 + 1/6 -1/7 + 1/7 -1/8 +1/8 -1/9 + 1/9 -1/10
= 1 - 1/10
= 9/10

3 câu trả lời 493

Câu hỏi hot cùng chủ đề

Bài viết mới nhất Lớp 4