tính : 91-5+91-4+91-1 =???
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HT
10 tháng 7 2016
\(A=\frac{\frac{2}{3}+\frac{2}{5}-\frac{2}{7}+\frac{2}{91}}{\frac{4}{3}+\frac{4}{5}-\frac{4}{7}+\frac{4}{91}}.\)
\(A=\frac{\frac{2}{3}+\frac{2}{5}-\frac{2}{7}+\frac{2}{91}}{2.\left(\frac{2}{3}+\frac{2}{5}-\frac{2}{7}+\frac{2}{91}\right)}\)
\(A=\frac{1}{2}\)
10 tháng 7 2016
\(A=\frac{\frac{2}{3}+\frac{2}{5}-\frac{2}{7}+\frac{2}{91}}{\frac{4}{3}+\frac{4}{5}-\frac{4}{7}+\frac{4}{91}}=\frac{\frac{2}{3}+\frac{2}{5}-\frac{2}{7}+\frac{2}{91}}{2.\left(\frac{2}{3}+\frac{2}{5}-\frac{2}{7}+\frac{2}{91}\right)}=\frac{1}{2}\)
Cách này là nhanh nhất rùi
Ủng hộ mk nha ^_-
LV
2
XO
2 tháng 5 2021
Ta có \(\frac{91}{1.4}+\frac{91}{4.7}+\frac{91}{7.10}+...+\frac{91}{88.91}=\frac{91}{3}.\left(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{88.91}\right)\)
\(=\frac{91}{3}.\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{88}-\frac{1}{91}\right)\)
\(=\frac{91}{3}.\left(1-\frac{1}{91}\right)=\frac{91}{3}.\frac{90}{91}=30\)
2 tháng 5 2021
9/1.4+9/4.7+9/7.10+…+9/88.91=30
=91/3. (3/1.4+3/4.7+3/7.10+…+3/88.91)
=91/3. (1-1/4+1/4-1/7+1/7-1/10+…+1/88-1/91)
=91/3*90/91=30
T
28 tháng 5 2019
#)Giải :
\(\frac{91}{1.4}+\frac{91}{4.7}+\frac{91}{7.11}+...+\frac{91}{88.91}\)
\(=\frac{91}{3}\left(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.11}+...+\frac{3}{88.91}\right)\)
\(=\frac{91}{3}\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+...+\frac{1}{88}-\frac{1}{91}\right)\)
\(=\frac{91}{3}\left(1-\frac{1}{91}\right)\)
\(=\frac{91}{3}.\frac{90}{91}=30\left(đpcm\right)\)
#~Will~be~Pens~#
DL
28 tháng 5 2019
\(\frac{91}{1\cdot4}+\frac{91}{4\cdot7}+...+\frac{91}{88\cdot91}=\frac{1}{3}\left(91-\frac{91}{4}+\frac{91}{4}-\frac{91}{7}+...-\frac{91}{91}\right)\)
\(=\frac{1}{3}\left(91-1\right)=\frac{1}{3}\cdot90=30\)
TM
0
PT
1
N
1
NX
8 tháng 8 2023
3A=5(3/1*4+3/4*7+...+3/91*94)
3A=5(1-1/4+1/4-1/7+...+1/91-1/94)
3A=5(1-1/94)
3A=5.93/94
A=(5.93/94):3
A=155/94
8 tháng 8 2023
Ta đặt
\(A=\dfrac{5}{1\times4}+\dfrac{5}{4\times7}+...+\dfrac{5}{91\times94}\)
\(\dfrac{3}{5}A=\dfrac{3}{1\times4}+\dfrac{3}{4\times7}+....+\dfrac{3}{91\times94}\)
\(\dfrac{3}{5}A=1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+...+\dfrac{1}{91}+\dfrac{1}{94}\)
\(\dfrac{3}{5}A=1-\dfrac{1}{94}=\dfrac{93}{94}\)
\(A=\dfrac{93}{94}\div\dfrac{3}{5}=\dfrac{155}{94}\)
KT
23 tháng 7 2018
\(\frac{0,5+\frac{2}{43}-\frac{2}{2017}}{\frac{3}{4}+\frac{3}{43}-\frac{3}{2017}}:\frac{\frac{4}{91}+\frac{4}{34}+\frac{4}{2017}}{\frac{5}{91}+\frac{5}{34}+\frac{5}{2017}}\)
\(=\frac{\frac{2}{4}+\frac{2}{43}-\frac{2}{2017}}{\frac{3}{4}+\frac{3}{43}-\frac{3}{2017}}:\frac{4\left(\frac{1}{91}+\frac{1}{34}+\frac{1}{2017}\right)}{5\left(\frac{1}{91}+\frac{1}{34}+\frac{1}{2017}\right)}\)
\(=\frac{2\left(\frac{1}{4}+\frac{1}{43}-\frac{1}{2017}\right)}{3\left(\frac{1}{4}+\frac{1}{43}-\frac{1}{2017}\right)}:\frac{4}{5}\)
\(=\frac{2}{3}.\frac{5}{4}=\frac{5}{6}\)
\(91-5+91-4+91-1\\=86+87+90\\=173+90\\=263\)
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