S=7/10+37/40+85/88+151/154+235/238.
mn tính giúp mik với ạ
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DK
1
TD
26 tháng 4 2020
\(A=\frac{12}{40}+\frac{12}{88}+\frac{12}{154}+\frac{12}{238}+\frac{12}{340}\)
\(A=\frac{12}{5\times8}+\frac{12}{8\times11}+\frac{12}{11\times14}+\frac{12}{14\times17}+\frac{12}{17.20}\)
\(A=4\times\left(\frac{3}{5\times8}+\frac{3}{8\times11}+\frac{3}{11\times14}+\frac{3}{14\times17}+\frac{3}{17\times20}\right)\)
\(A=4\times\left(\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}+\frac{1}{14}-\frac{1}{17}+\frac{1}{17}-\frac{1}{20}\right)\)
\(A=4\times\left(\frac{1}{5}-\frac{1}{20}\right)\)
\(A=4\times\frac{3}{20}=\frac{12}{20}=\frac{3}{5}\)
LT
1
28 tháng 7 2015
\(S=\frac{1}{10}+\frac{1}{40}+\frac{1}{88}+\frac{1}{184}+\frac{1}{238}+\frac{1}{340}=\frac{1}{3}\left(\frac{3}{2.5}+\frac{3}{5.8}+\frac{3}{8.11}+\frac{3}{11.14}+\frac{3}{14.17}+\frac{3}{17.20}\right)\)
\(=\frac{1}{3}\left(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}+\frac{1}{14}-\frac{1}{17}+\frac{1}{17}-\frac{1}{20}\right)\)
\(=\frac{1}{3}\left(\frac{1}{2}-\frac{1}{20}\right)=\frac{1}{3}.\frac{9}{20}=\frac{3}{20}>\frac{2}{20}=\frac{1}{10}=0,1\)
vậy S>0,1
NP
2
HT
28 tháng 7 2015
S = \(\frac{1}{10}+\frac{1}{40}+\frac{1}{88}+\frac{1}{154}+\frac{1}{238}+\frac{1}{340}\)
S = \(\frac{1}{2.5}+\frac{1}{5.8}+\frac{1}{8.11}+\frac{1}{11.14}+\frac{1}{14.17}+\frac{1}{17.20}\)
S = \(\frac{1}{3}\left(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+....+\frac{1}{17}-\frac{1}{20}\right)\)
S = \(\frac{1}{3}\left(\frac{1}{2}-\frac{1}{20}\right)=\frac{1}{3}.\frac{9}{20}\)
S = \(\frac{3}{20}\)
S = 0,15 > 0,1
TT
7
DK
0
TN
4
LK
1
\(S=\frac{7}{10}+\frac{37}{40}+\frac{85}{88}+\frac{151}{154}+\frac{235}{238}\)
\(=1-\frac{3}{10}+1-\frac{3}{40}+1-\frac{3}{88}+1-\frac{3}{154}+1-\frac{3}{238}\)
\(=5-\left(\frac{3}{2\times5}+\frac{3}{5\times8}+\frac{3}{8\times11}+\frac{3}{11\times14}+\frac{3}{14\times17}\right)\)
\(=5-\left(\frac{5-2}{2\times5}+\frac{8-5}{5\times8}+\frac{11-8}{8\times11}+\frac{14-11}{11\times14}+\frac{17-14}{14\times17}\right)\)
\(=5-\left(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}+\frac{1}{14}-\frac{1}{17}\right)\)
\(=5-\left(\frac{1}{2}-\frac{1}{17}\right)=\frac{155}{34}\).