Tính nhanh:
a. 3/4x9 + 3/9*14 +...+ 3/54x59 + 3/59x64.
b. 2/8x11 + 2/11x14 +...+ 2/23x26 + 2/26x29
K
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PT
c) C=2 x5+5x8+8x11+...+23x26+26x29
d) D=3x7+7x11+11x15+...+43x47+47x51
giúp mik với thank các bn nhiều
0
BD
15 tháng 8 2023
\(B=\dfrac{3}{2\cdot5}+\dfrac{3}{5\cdot8}+\dfrac{3}{8\cdot11}+...+\dfrac{3}{26\cdot29}\)
\(B=\dfrac{1}{2}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{11}+...+\dfrac{1}{26}-\dfrac{1}{29}\)
\(B=\dfrac{1}{2}-\dfrac{1}{29}\)
\(B=\dfrac{27}{58}\)
15 tháng 8 2023
B= 3/2x5 + 3/5x8+ 3/8x11 + ... + 3/26x29
B= 1/2 - 1/5 + 1/5 - 1/8 + 1/8 - 1/11 + ... + 1/26 - 1/29
B= 1/2-1/29
B=27/58
DT
2
N
24 tháng 6 2017
\(E=\frac{3^2}{8.11}+\frac{3^2}{11.14}+......+\frac{3^2}{197.200}\)
\(E=3.\left(\frac{3}{8.11}+\frac{3}{11.14}+......+\frac{3}{197.200}\right)\)
\(E=3.\left(\frac{1}{8}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}+.....+\frac{1}{197}-\frac{1}{200}\right)\)
\(E=3.\left(\frac{1}{8}-\frac{1}{200}\right)\)
\(E=3.\frac{3}{25}\)
\(E=\frac{9}{25}\)
24 tháng 6 2017
\(\frac{1}{3}E=\frac{3}{8x11}+\frac{3}{11x14}+..+\frac{3}{197x200}\)
\(\frac{1}{3}E=\frac{1}{8}-\frac{1}{11}+\frac{1}{11}-\frac{1}{13}+.+\frac{1}{197}-\frac{1}{200}\)
\(\frac{1}{3}E=\frac{1}{8}-\frac{1}{200}\)
\(\frac{1}{3}E=\frac{3}{25}\)
\(E=\frac{9}{25}\)
17 tháng 8 2016
\(A=\frac{2}{1.5}+\frac{2}{5.9}+\frac{2}{9.13}+....+\frac{2}{81.85}\)
\(\Rightarrow2A=\frac{4}{1.5}+\frac{4}{5.9}+\frac{4}{9.13}+....+\frac{4}{81.85}\)
\(\Rightarrow2A=1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+....+\frac{1}{81}-\frac{1}{85}\)
\(\Rightarrow2A=1-\frac{1}{85}\)
\(\Rightarrow A=\frac{84}{85}:2=\frac{42}{85}\)
17 tháng 8 2016
tính A còn lại tự tính nha
a) A = 2/1x5 + 2/5x9 + 2/9x13 +....+2/81x85
\(\frac{2}{1x5}+\frac{2}{5x9}+\frac{2}{9x13}+...+\frac{2}{81x85}\)
\(A=1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+...+\frac{1}{81}-\frac{1}{85}\)
\(A=1-\frac{1}{85}\)
\(\Rightarrow A=\frac{84}{85}\)
k nha
KD
4
14 tháng 5 2018
\(=\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}\)
\(=\frac{1}{2}-\frac{1}{14}\)
\(=\frac{7}{14}-\frac{1}{14}\)
\(=\frac{6}{14}\)
\(=\frac{3}{7}\)
14 tháng 5 2018
3/2x5 + 3/5x8 + 3/8x11 + 3/11x14
= 3/2 - 3/5 + 3/5 - 3/8 + 3/8 - 3/11 + 3/11 - 3/14
= 3/2 - 3/14
= 21/14 - 3/14
= 18/14
= 9/5
VT
28 tháng 3 2022
A = \(\dfrac{1}{2}\) - \(\dfrac{1}{5}\) + \(\dfrac{1}{5}\) - \(\dfrac{1}{8}\) + \(\dfrac{1}{8}\) - \(\dfrac{1}{11}\) + \(\dfrac{1}{11}\) - \(\dfrac{1}{14}\) + \(\dfrac{1}{14}\) - \(\dfrac{1}{17}\) + \(\dfrac{1}{17}\) - \(\dfrac{1}{20}\)
= \(\dfrac{1}{2}\) - \(\dfrac{1}{20}\)
= \(\dfrac{9}{20}\)
16 tháng 8 2017
= 5-2/2x5+8-5/5x8+11-8/8x11+14-11/11x14
=(1/2-1/5)+(1/5-1/8)+(1/8-1/11)+(1/11-1/14)
=(1/2+1/5+1/8+1/11)-(1/5+1/8+1/11+1/14)
=1/2-1/14
=3/7
Vậy B=3/7
T
3
10 tháng 7 2019
\(A=\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+...+\frac{1}{11}-\frac{1}{14}\)
\(A=\frac{1}{2}-\frac{1}{14}\)
\(A=\frac{3}{7}\)
10 tháng 7 2019
\(A=\)\(\frac{3}{5\times2}+\frac{3}{5\times8}+\frac{3}{8\times11}+...+\frac{3}{11\times14}\)
\(=\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+...+\frac{1}{11}-\frac{1}{14}\)
\(=\frac{1}{2}-\frac{1}{14}\)
\(=\frac{3}{7}\)
a) \(\frac{3}{4\times9}+\frac{3}{9\times14}+...+\frac{3}{54\times59}+\frac{3}{59\times64}\)
\(=\frac{3}{5}\times\left(\frac{5}{4\times9}+\frac{5}{9\times14}+...+\frac{5}{59\times64}\right)\)
\(=\frac{3}{5}\times\left(\frac{9-4}{4\times9}+\frac{14-9}{9\times14}+...+\frac{64-59}{59\times64}\right)\)
\(=\frac{3}{5}\times\left(\frac{1}{4}-\frac{1}{9}+\frac{1}{9}-\frac{1}{14}+...+\frac{1}{59}-\frac{1}{64}\right)\)
\(=\frac{3}{5}\times\left(\frac{1}{4}-\frac{1}{64}\right)\)
\(=\frac{9}{64}\)
b) \(\frac{2}{8\times11}+\frac{2}{11\times14}+...+\frac{2}{23\times26}+\frac{2}{26\times29}\)
\(=\frac{2}{3}\times\left(\frac{3}{8\times11}+\frac{3}{11\times14}+...+\frac{3}{26\times29}\right)\)
\(=\frac{2}{3}\times\left(\frac{11-8}{8\times11}+\frac{14-11}{11\times14}+...+\frac{29-26}{26\times29}\right)\)
\(=\frac{2}{3}\times\left(\frac{1}{8}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}+...+\frac{1}{26}-\frac{1}{29}\right)\)
\(=\frac{2}{3}\times\left(\frac{1}{8}-\frac{1}{29}\right)\)
\(=\frac{7}{116}\)