b= 1/25x27 + 1/27x29 + 1/29x31 + ...+ 1/93x95
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ML
19 tháng 7 2017
\(B=\frac{1}{25.27}+\frac{1}{27.29}+\frac{1}{29.31}+...+\frac{1}{73.75}\)
\(B=\frac{1}{2}\left(\frac{1}{25}-\frac{1}{27}+\frac{1}{27}-\frac{1}{29}+\frac{1}{29}-\frac{1}{31}+...+\frac{1}{73}-\frac{1}{75}\right)\)
\(B=\frac{1}{2}\left(\frac{1}{25}-\frac{1}{75}\right)\)
\(B=\frac{1}{2}\cdot\frac{2}{75}\)
\(B=\frac{1}{75}\)
DP
19 tháng 7 2017
\(B=\frac{1}{25.27}+\frac{1}{27.29}+\frac{1}{29.31}+....+\frac{1}{73.75}\)
\(B=\frac{1}{2}.\left(\frac{1}{25}-\frac{1}{27}+\frac{1}{27}-\frac{1}{29}+\frac{1}{29}-\frac{1}{31}+...+\frac{1}{73}-\frac{1}{75}\right)\)
\(B=\frac{1}{2}\cdot\left(\frac{1}{25}-\frac{1}{75}\right)\)
\(B=\frac{1}{2}\cdot\frac{2}{75}=\frac{1}{75}\)
TM
20 tháng 6 2016
\(\left(\frac{1}{25x27}+\frac{1}{27x29}+\frac{1}{29x31}+....+\frac{1}{73x75}\right)x\frac{3}{10}=x\)
\(< =>\frac{1}{2}x\left(\frac{2}{25x27}+\frac{2}{27x29}+\frac{2}{29x31}+....+\frac{2}{73x75}\right)x\frac{3}{10}=x\)
\(< =>\frac{1}{2}x\left(\frac{1}{25}-\frac{1}{27}+\frac{1}{27}-\frac{1}{29}+\frac{1}{29}-\frac{1}{31}+....+\frac{1}{73}-\frac{1}{75}\right)x\frac{3}{10}=x\)
\(< =>\frac{1}{2}x\left(\frac{1}{25}-\frac{1}{75}\right)x\frac{3}{10}=x< =>\frac{1}{2}x\frac{2}{25}x\frac{3}{10}=x< =>x=\frac{3}{250}\)
11 tháng 4 2017
D=6/15.18+6/18.21+...+6/89.92
D=6(1/15.18+1/18.21+...+1/89.92)
a) 3D=6(1/15-1/18+1/18-1/21+...+1/89-1/92)
3D=6(1/15-1/92)
3D=6.(77/1380)
3D=77/230
D=77/690
b) F=1/25.27+1/27.29+...+1/73.75
2F=2/25.27+2/27.29+..+2/73.75
2F=1/25-1/27+1/27-1/29+...+1/73-1/75
2F=1/25-1/75
2F=2/75
F=1/75
TT
2
N
15 tháng 7 2017
\(\frac{1}{25.27}+\frac{1}{27.29}+.......+\frac{1}{77.79}\)
\(=\frac{1}{2}.\left(\frac{2}{25.27}+\frac{2}{27.29}+....+\frac{2}{77.79}\right)\)
\(=\frac{1}{2}.\left(\frac{1}{25}-\frac{1}{27}+\frac{1}{27}-\frac{1}{29}+......+\frac{1}{77}+\frac{1}{79}\right)\)
\(=\frac{1}{2}.\left(\frac{1}{25}-\frac{1}{79}\right)\)
\(=\frac{1}{2}.\frac{54}{1975}\)
\(=\frac{27}{1975}\)
15 tháng 7 2017
\(A=\frac{1}{25.27}+\frac{1}{27.29}+...........+\frac{1}{75.79}\)
\(\Leftrightarrow2A=\frac{2}{25.27}+\frac{2}{27.29}+........+\frac{2}{75.79}\)
\(\Leftrightarrow2A=\frac{1}{25}-\frac{1}{27}+\frac{1}{27}-\frac{1}{29}+.........+\frac{1}{75}-\frac{1}{79}\)
\(\Leftrightarrow2A=\frac{1}{25}-\frac{1}{79}\)
Tự tính tiếp! chúc b hk tốt
LB
25 tháng 3 2017
\(A=\frac{1}{25.27}+\frac{1}{27.29}+...+\frac{1}{73.75}=\frac{1}{2}\left(\frac{1}{25}-\frac{1}{27}+\frac{1}{27}-\frac{1}{29}+...+\frac{1}{73}-\frac{1}{75}\right)\)
\(A=\frac{1}{2}\left(\frac{1}{25}-\frac{1}{75}\right)\\ A=\frac{1}{75}\)
\(B=\frac{15}{90.94}+\frac{15}{94.98}+...+\frac{15}{146+150}=\frac{1}{4}\left(\frac{15}{90}-\frac{15}{94}+\frac{15}{94}-\frac{15}{98}+...+\frac{15}{146}-\frac{15}{150}\right)\)
\(B=\frac{1}{4}\left(\frac{15}{90}-\frac{15}{150}\right)=\frac{1}{60}\)
LH
1 tháng 8 2016
\(B=1.3+3.5+5.7+....+27.29+29.31\)
\(6B=1.3.6+3.5.6+5.7.6+....+27.29.6+29.31.6\)
\(6B=1.3.\left(5+1\right)+3.5.\left(7-1\right)+5.7.\left(9-3\right)+...+27.29.\left(31-25\right)+29.31.\left(33-27\right)\)
\(6B=1.3.5+3+3.5.7-1.3.5+5.7.9-3.5.7+...+27.29.31-25.27.29+29.31.33-27.29.31\)
\(6B=3+29.31.33\)
\(B=\frac{3+29.21.33}{6}=4945\)
11 tháng 8 2019
a) A = 9/8.11 + 9/11.14 + 9/14.17 + ... + 9/73.75
A = 3.(1/8 - 1/11 + 1/11 - 1/14 + 1/14 - 1/17 + ... + 1/73 - 1/75)
A = 3.(1/8 - 1/75)
A = 3.67/600
A = 67/200
Các bài sau làm tương tự, riêng câu D thì phân tích ra
11 tháng 8 2019
Mình chỉ làm hộ bạn câu a) thôi nhé vì đề sàn sàn giống nhau :
a) \(A=\frac{9}{8×11}+\frac{9}{11×14}+\frac{9}{14×17}+...+\frac{9}{73×75}\)
\(A=\frac{9}{8}-\frac{9}{11}+\frac{9}{11}-\frac{9}{14}+\frac{9}{14}-\frac{9}{17}+...+\frac{9}{73}-\frac{9}{75}\)
\(A=\frac{9}{8}-\frac{9}{75}\)
\(A=\frac{675}{600}-\frac{72}{600}\)
\(A=\frac{673}{600}\)
Vậy,...
Cbht
\(B=\dfrac{1}{25.27}+\dfrac{1}{27.29}+\dfrac{1}{29.31}+...+\dfrac{1}{93.95}\)
\(B=\dfrac{1}{2}.\left(\dfrac{1}{25}-\dfrac{1}{27}+\dfrac{1}{27}-\dfrac{1}{29}+\dfrac{1}{29}-\dfrac{1}{31}+...+\dfrac{1}{93}-\dfrac{1}{95}\right)\)
\(B=\dfrac{1}{2}.\left(\dfrac{1}{25}-\dfrac{1}{95}\right)\)
\(B=\dfrac{1}{2}.\dfrac{14}{475}\)
\(B=\dfrac{7}{475}\)
\(#NqHahh\)
\(B=\dfrac{1}{25\times27}+\dfrac{1}{27\times29}+\dfrac{1}{29\times31}+...+\dfrac{1}{93\times95}\)
\(B=\dfrac{1}{2}\times\left(\dfrac{1}{25\times27}+\dfrac{1}{27\times29}+\dfrac{1}{29\times31}+...+\dfrac{1}{93\times95}\right)\)
\(B=\dfrac{1}{2}\times\left(\dfrac{1}{25}-\dfrac{1}{27}+\dfrac{1}{27}-\dfrac{1}{29}+...+\dfrac{1}{93}-\dfrac{1}{95}\right)\)
\(B=\dfrac{1}{2}\times\left(\dfrac{1}{25}-\dfrac{1}{95}\right)\)
\(B=\dfrac{1}{2}\times\left(\dfrac{95}{2375}-\dfrac{25}{2375}\right)\)
\(B=\dfrac{1}{2}\times\dfrac{14}{475}\)
\(B=1\times\dfrac{7}{475}\)
\(B=\dfrac{7}{475}\)