1/2 + 1/6 + 1/12 +......+ 1/72 + 1/90
Giúp mình nhé😘
K
Khách
Hãy nhập câu hỏi của bạn vào đây, nếu là tài khoản VIP, bạn sẽ được ưu tiên trả lời.
Những câu hỏi liên quan
NT
1
VD
18 tháng 3 2022
\(B=\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}+\dfrac{1}{72}+\dfrac{1}{90}\\ =\dfrac{1}{4\times5}+\dfrac{1}{5\times6}+\dfrac{1}{6\times7}+\dfrac{1}{7\times8}+\dfrac{1}{8\times9}+\dfrac{1}{9\times10}\\ =\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}\\ =\dfrac{1}{4}-\dfrac{1}{10}\\ =\dfrac{10}{40}-\dfrac{4}{40}\\ =\dfrac{6}{40}\\ =\dfrac{3}{20}\)
1 tháng 11 2016
\(\frac{8}{9}-\frac{1}{72}-\frac{1}{56}-...-\frac{1}{12}-\frac{1}{6}-\frac{1}{2}\)
\(=\frac{8}{9}-\left(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{56}+\frac{1}{72}\right)\)
\(=\frac{8}{9}-\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{7.8}+\frac{1}{8.9}\right)\)
\(=\frac{8}{9}-\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}\right)\)
\(=\frac{8}{9}-\left(1-\frac{1}{9}\right)\)
\(=\frac{8}{9}-\frac{8}{9}=0\)
VH
1
22 tháng 7 2019
https://olm.vn/hoi-dap/detail/7709875367.html
VT
15 tháng 5 2018
= 1- ( 1/2 +1/6 +1/12+1/20+1/30+1/42+1/56+1/72+1/89)
=1-721/801
= 80/801
15 tháng 5 2018
Đề hơi
sai !!
SỬa số 1/89 thành 1/90 hình như ms đúng quy luật !!
22 tháng 3 2018
ta có: 1/2=1/1x2
1/6=1/2x3
1/12=1/3x4
1/20=1/4x5
1/30=1/5x6
1/42=1/6x7
1/56=1/7x8
1/72=1/8x9
1/90=1/9x10
1/110=1/10x11
tiếp theo bn tiếp tục nhé
22 tháng 3 2018
1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90+1/110
=1/1x2+1/2x3+1/3x4+1/4x5+1/5x6+1/6x7+1/7x8+1/8x9+1/9x10+1/10x1
=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+1/10-1/11
=1-1/11
=10/11
4 tháng 7 2019
= 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9
= 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+1/9
=1-1/9
=8/9
4 tháng 7 2019
\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}\)
=\(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}\)
= \(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}\)
= \(1-\frac{1}{9}\)
= \(\frac{8}{9}\)
LH
2
20 tháng 5 2021
1/6+1/12+....+1/90
= 1/2.3+1/3.4+1/4.5+.....+1/9.10
= 1/2-1/3+1/3-1/4+1/4-.......+1/9-1/10
= 1/2-1/10
=2/5
dấu chấm là nhân nha
20 tháng 5 2021
1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 +1/90
=1/2.3 + 1/3.4 +1/4.5 +1/5.6 +1/6.7 +1/7.8 +1/8.9
=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
=1/2 -1/9
=9/18 - 2/18
=7/18
NM
2
18 tháng 7 2020
Tính
\(D=\frac{5}{1.6}+\frac{5}{6.11}+\frac{5}{11.16}+...+\frac{5}{86.91}\)
\(=1-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+\frac{1}{11}-\frac{1}{16}+...+\frac{1}{86}-\frac{1}{91}\)
\(=1-\frac{1}{91}=\frac{91}{91}-\frac{1}{91}=\frac{90}{91}\)
Vậy \(D=\frac{90}{91}.\)
T
18 tháng 7 2020
\(D=\frac{5}{1.6}+\frac{5}{6.11}+\frac{5}{11.16}+...+\frac{5}{86.91}\)
\(=1-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+\frac{1}{11}-\frac{1}{16}+....+\frac{1}{86}-\frac{1}{91}\)
\(=1-\frac{1}{91}\)
\(=\frac{91}{91}-\frac{1}{91}\)
\(=\frac{90}{91}\)
Vậy \(D=\frac{90}{91}\)
Chúc bạn học tốt !
NA
Ngoc Anh Thai
Giáo viên
28 tháng 3 2021
a)
\(\dfrac{2}{3}x-\dfrac{1}{5}x=-\dfrac{7}{10}\\ \left(\dfrac{2}{3}-\dfrac{1}{5}\right)x=-\dfrac{7}{10}\\ \dfrac{7}{15}x=-\dfrac{7}{10}\\ x=-\dfrac{7}{10}:\dfrac{7}{15}\\ x=-\dfrac{15}{10}\\ x=-\dfrac{3}{2}.\)
b)
\(\left(\dfrac{1}{10}-1\right).\left(\dfrac{1}{11}-1\right).\left(\dfrac{1}{12}-1\right)...\left(\dfrac{1}{2021}-1\right)\\ =\dfrac{-9}{10}.\dfrac{-10}{11}.\dfrac{-11}{12}...\dfrac{-2020}{2021}\\ =\dfrac{9}{2021}.\)
c)
\(\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}+\dfrac{1}{72}+\dfrac{1}{90}\\ =\dfrac{1}{3.4}+\dfrac{1}{4.5}+\dfrac{1}{5.6}+\dfrac{1}{6.7}+\dfrac{1}{7.8}+\dfrac{1}{8.9}+\dfrac{1}{9.10}\\ =\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{9}-\dfrac{1}{10}\\ =\dfrac{1}{3}-\dfrac{1}{10}\\ =\dfrac{7}{30}.\)
(câu c em ghi nhầm đề 1 chút nhé, chỗ đó là 56 chứ không phải 50)
Đề: Tính
\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{72}+\frac{1}{90}\)
\(=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{8.9}+\frac{1}{9.10}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\)
\(=1-\frac{1}{10}=\frac{10}{10}-\frac{1}{10}=\frac{9}{10}\)
\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+....+\frac{1}{72}+\frac{1}{90}\)
\(=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+......+\frac{1}{8.9}+\frac{1}{9.10}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+....+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\)
\(=1-\frac{1}{10}\)
\(=\frac{10}{10}-\frac{1}{10}\)
\(=\frac{9}{10}\)
Học tốt