Tính bằng cách hợp lí nhất : \(\frac{2}{7}\times5\frac{1}{4}-\frac{2}{7}\times3\frac{1}{4}\)
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
24 tháng 3 2018
Suy ra 2A=2/1x2x3+2/2x3x4+2/3x4x5+......+2/38x39x40
2A=3-1/1x2x3+4-2/2x3x4+5-3/3x4x5+........+40-38/38x39x40
2A=1/1x2-1/2x3+1/2x3-1/3x4+1/4x5-1/5x6+........+1/38x39-1/39x40
2A=1/2-1/1560
2A=780/1560-1/1560
2A=779/1560
A=779/1560:2
A=779/1560x1/2
A=779/3120
24 tháng 3 2018
\(A=\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+.......+\frac{1}{38.39.40}\)
\(2A=\frac{2}{1.2.3}+\frac{2}{2.3.4}+\frac{2}{3.4.5}+.........+\frac{2}{38.39.40}\)
\(2A=\frac{3-1}{1.2.3}+\frac{4-2}{2.3.4}+\frac{5-3}{3.4.5}+....+\frac{40-38}{38.39.40}\)
\(2A=\frac{3}{1.2.3}-\frac{1}{1.2.3}+\frac{4}{2.3.4}-\frac{2}{2.3.4}+\frac{5}{3.4.5}-\frac{3}{3.4.5}+.......+\frac{40}{38.39.40}-\frac{38}{38.39.40}\)
\(2A=\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+\frac{1}{3.4}-\frac{1}{4.5}+.......+\frac{1}{38.39}-\frac{1}{39.40}\)
\(2A=\frac{1}{1.2}-\frac{1}{39.40}\)
\(2A=\frac{1}{2}-\frac{1}{1560}\)
\(2A=\frac{779}{1560}\)
\(A=\frac{779}{1560}:2\)
\(A=\frac{779}{3120}\)
1 tháng 5 2015
\(\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{8.9.10}=\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{2.3}\right)+\frac{1}{2}.\left(\frac{1}{2.3}-\frac{1}{3.4}\right)+...+\frac{1}{2}.\left(\frac{1}{8.9}-\frac{1}{9.10}\right)\)
\(=\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{8.9}-\frac{1}{9.10}\right)\)
\(\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{9.10}\right)=\frac{1}{2}.\frac{22}{45}=\frac{11}{45}\)
NN
Nguyễn Ngọc Anh Minh
CTVHS
VIP
28 tháng 8 2018
\(3C=\frac{3}{1.2.3.4}+\frac{3}{2.3.4.5}+...+\frac{3}{27.28.29.30}\)
\(3C=\frac{4-1}{1.2.3.4}+\frac{5-2}{2.3.4.5}+...+\frac{30-27}{27.28.29.30}\)
\(3C=\frac{1}{1.2.3}-\frac{1}{2.3.4}+\frac{1}{2.3.4}-\frac{1}{3.4.5}+...+\frac{1}{27.28.29}+\frac{1}{28.29.30}\)
\(3C=\frac{1}{1.2.3}-\frac{1}{28.29.30}\Rightarrow C=\left(\frac{1}{1.2.3}-\frac{1}{28.29.30}\right):3\)
NC
16 tháng 8 2020
a) \(\frac{7^3.5^8}{49.25^4}=\frac{7^3.5^8}{7^2.5^8}=7\)
b) \(\frac{3^9.25.5^3}{15.625.3^8}=\frac{3^9.5^2.5^3}{3.5.5^4.3^8}=\frac{3^9.5^5}{3^9.5^5}=1\)
c) \(\frac{2^{50}.3^{61}+2^{90}.3^{16}}{2^{51}.3^{61}+2^{91}.3^{16}}=\frac{2^{50}.3^{16}\left(3^{45}+2^{40}\right)}{2^{51}.3^{16}\left(3^{45}+2^{40}\right)}=\frac{1}{2}\)
d) \(\left(\frac{2}{5}-\frac{1}{2}\right)^2+\left(\frac{1}{2}+\frac{3}{5}\right)^2\)
\(=\left(\frac{-1}{10}\right)^2+\left(\frac{11}{10}\right)^2\)
\(=\frac{1}{100}+\frac{121}{100}=\frac{122}{100}=\frac{61}{50}\)
Q
11 tháng 4 2017
\(\frac{-3}{5}.\frac{4}{7}+\frac{-3}{5}.\frac{2}{7}+\frac{-3}{5}\)
\(=\frac{-3}{5}.\frac{4}{7}+\frac{-3}{5}.\frac{2}{7}+\frac{-3}{5}.\frac{1}{7}\)
\(=\frac{-3}{5}\left(\frac{4}{7}+\frac{2}{7}+\frac{1}{7}\right)\)
\(=\frac{-3}{5}.1\)
\(=\frac{-3}{5}\)
11 tháng 4 2017
\(-\frac{2}{5}x\frac{1}{7}+-\frac{3}{5}x\frac{2}{7}+-\frac{3}{5}=-\frac{2}{5}x\frac{1}{7}+-\frac{3}{5}\left(\frac{2}{7}+1\right)=-\frac{2}{5}x\frac{1}{7}+-\frac{3}{5}x\frac{9}{7}.\)
\(-\frac{2}{35}-\frac{27}{35}=-\frac{29}{35}\)
tôi tính nhẩm vậy ko biết có đứng ko
23 tháng 4 2016
1/2.3+1/3.4+1/4.5+1/5.6+1/6.7+1/7.8+1/8.9+1/9.10
=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
=1/2-1/10
=5/10-1/10
=4/10=2/5
23 tháng 4 2016
\(\frac{1}{2x3}+\frac{1}{3x4}+\frac{1}{4x5}+\frac{1}{5x6}+\frac{1}{6x7}+\frac{1}{8x9}+\frac{1}{9x10}\)
\(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\)
\(\frac{1}{2}-\frac{1}{10}\)
\(\frac{2}{5}\)
BL
25 tháng 9 2017
1 x 3 x 5 + 2 x 6 x 10 + 4 x 10 x 12 + 7 x 21 x 35 / 1 x 5 x 7 + 2 x 10 x 14 + 4 x 20 x 28 + 7 x 35 x 49
= 3 / 7 + 6 / 14 + 10 x 2 x 6 / 10 x 2 x 28 + 21 / 49
= 3 / 7 + 6 / 14 + 6 / 28 + 21 / 49
= 3 / 7 + 6 / 14 + 3 / 14 + 3 / 7
= ( 3 / 7 + 3 / 7 ) + ( 6 / 14 + 3 / 14 )
= 6 / 7 + 9 14
= 12 / 14 + 9 / 14
= 21 / 14
nho hem
lm dau tien lun do
dung 100% nha
\(\frac{2}{7}.5\frac{1}{4}-\frac{2}{7}.3\frac{1}{4}\)
= \(\frac{2}{7}.\left(5\frac{1}{4}-3\frac{1}{4}\right)\)
= \(\frac{2}{7}.2\)
= \(\frac{4}{7}\)
\(=\frac{2}{7}\times\left(5\frac{1}{4}-3\frac{1}{4}\right)=\frac{2}{7}\times2=\frac{4}{7}\)