Tính thuận tiện nhất
2/7×9 + 2/9×11 + 2/11×13 ....+ 2/75×77
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VN
Võ Ngọc Phương
CTVHS
VIP
24 tháng 8 2023
\(\dfrac{5}{13}\times\dfrac{7}{11}+\dfrac{5}{13}\times\dfrac{2}{11}+\dfrac{8}{13}\times\dfrac{9}{11}\)
\(=\dfrac{5}{13}\times\left(\dfrac{7}{11}+\dfrac{2}{11}\right)+\dfrac{8}{13}\times\dfrac{9}{11}\)
\(=\dfrac{5}{13}\times\dfrac{9}{11}+\dfrac{8}{13}\times\dfrac{9}{11}\)
\(=\dfrac{9}{11}\times\left(\dfrac{5}{13}+\dfrac{8}{13}\right)\)
\(=\dfrac{9}{11}\times1\)
\(=\dfrac{9}{11}\)
VN
Võ Ngọc Phương
CTVHS
VIP
24 tháng 8 2023
a) \(20,23\times21+20,23\times80-20,23\)
\(=20,23\times\left(21+80-1\right)\)
\(=20,23\times100\)
\(=2023\)
NT
4
TM
15 tháng 5 2016
a,3/5*7/9+11/9*3/5-3/10=3/5*(7/9+11/9)-3/10
=3/5*18/9-3/10
=3/5*2-3/10
=6/5-3/10
=12/10-3/10
=9/10
b,14/13:1/2+3/13*2-2*7/13=14/13*2+3/13*2-2*7/13
=2*(14/13+3/13-7/13)
=2*10/13
=20/13
15 tháng 5 2016
a/ ( 7/9 + 11/9 ) x 3/5 - 3/10
= 2 x 3/5 - 3/10
= 6/5 - 3/10
= 9/10
b/ 14/13 : 1/2 + 3/13 x 2 - 2 x 7/13
14/13 x 2/1 + 3/13 x 2 - 2 x 7/13
2 x ( 14/13 + 3/13 - 7/13 )
2 x 10/13
20/13
NK
6
VT
0
NQ
0
24 tháng 4 2021
a) \(1678\cdot24+77\cdot1678-1678=1678\left(24+77-1\right)=167800\)
24 tháng 4 2021
b) Ta có: \(\dfrac{2}{5}+\dfrac{4}{11}+\dfrac{3}{5}+\dfrac{18}{11}+\dfrac{1}{4}+\dfrac{1}{6}\)
\(=\left(\dfrac{2}{5}+\dfrac{3}{5}\right)+\left(\dfrac{4}{11}+\dfrac{18}{11}\right)+\dfrac{1}{4}+\dfrac{1}{6}\)
\(=3+\dfrac{1}{4}+\dfrac{1}{6}\)
\(=\dfrac{36}{12}+\dfrac{3}{12}+\dfrac{2}{12}=\dfrac{41}{12}\)
TK
2
26 tháng 4 2017
A)\(\frac{3}{5}\times\frac{7}{9}+\frac{11}{9}\times\frac{3}{5}-\frac{3}{10}\\ =\frac{3}{5}\times\left(\frac{7}{9}+\frac{11}{9}\right)-\frac{3}{10}\\ =\frac{3}{5}\times\frac{18}{9}-\frac{3}{10}=\frac{3\times2}{5}-\frac{3}{10}\)
\(=\frac{6}{5}-\frac{3}{10}=\frac{12}{10}-\frac{3}{10}=\frac{9}{10}\)
TT
4
15 tháng 5 2018
=(1/11+10/11)+(2/11+9/11)+(3/11+8/11)+(4/11+7/11)+(5/11+6/11)
=11/11+11/11+11/11+11/11+11/11
=1+1+1+1+1
=5
\(\dfrac{2}{7\times9}+\dfrac{2}{9\times11}+\dfrac{2}{11\times13}+...+\dfrac{2}{75\times77}\)
\(=\dfrac{1}{7}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{13}+...+\dfrac{1}{75}-\dfrac{1}{77}\)
\(=\dfrac{1}{7}-\dfrac{1}{77}\)
\(=\dfrac{11}{77}-\dfrac{1}{77}\)
\(=\dfrac{10}{77}\)
\(\dfrac{2}{7x9}+\dfrac{2}{9x11}+\dfrac{2}{11x13}+...+\dfrac{2}{75x77}\)
= \(\dfrac{9-7}{7x9}+\dfrac{11-9}{9x11}+\dfrac{13-11}{11x13}+...+\dfrac{77-75}{75x77}\)
= \(\dfrac{9}{7x9}-\dfrac{7}{7x9}+\dfrac{11}{9x11}-\dfrac{9}{9x11}+\dfrac{13}{11x13}-\dfrac{11}{11x13}+...+\dfrac{77}{75x77}-\dfrac{75}{75x77}\)
=\(\dfrac{1}{7}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{13}+...+\dfrac{1}{75}-\dfrac{1}{77}\)
=\(\dfrac{1}{7}-\dfrac{1}{77}=\dfrac{11}{77}-\dfrac{1}{77}=\dfrac{10}{77}\)