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b1: a, 612.(15+19-34)=612.0=0
b,414.(37.4+23.4-240)=414.0=0
c,(517.125-518.25)+63:23=(517.53-518.52)+33=0+27=27
b2:a,143+7.(n-17)=206
===> 7.(n-17)=206-143=63
====>n-17=63:7=9
=====>n=9+17=26
vậy n=26
b,128-28:(15-n)=124
====>28:(15-n)=128-124=4
=====> 15-n=28:4=7
=====> n=15-7=8
vậy n=8
c,3n.2+48=210
====>3n.2=210-48=162
====>3n=162:2=81=34
====>n=4
vậy n=4
Ta có:
\(B=\frac{1}{4}+\frac{1}{5}+\frac{1}{6}+...+\)\(\frac{1}{19}\)
\(B=\left(\frac{1}{4}+\frac{1}{5}+\frac{1}{6}+...+\frac{1}{15}\right)+\left(\frac{1}{16}+...+\frac{1}{19}\right)\)
\(\Rightarrow B>\left(\frac{1}{15}+\frac{1}{15}+\frac{1}{15}+...+\frac{1}{15}\right)+\left(\frac{1}{20}+...+\frac{1}{20}\right)\)
\(B>\frac{4}{5}+\frac{1}{5}\)
\(B>1\)\(\left(đpcm\right)\)
Ta có
\(\frac{2}{3}.x=5\)
\(\Rightarrow x=5:\frac{2}{3}\)
\(\Rightarrow x=\frac{15}{2}\)
Chúc bạn học tốt!
S=3/2^0+3/2^1+....+3/2^2018
S=3/2.(2/2^0+2/2^1+....+2^2018)
đặt B=2/2^0+2/2^1+....+2^2018
2B=2.(2/2^0+2/2^1+....+2^2018)
2B=1+2/2^0+...+2/2^2017
2B-B=(1+2/2^0+...+2/2^2017)-(2/2^0+2/2^1+....+2^2018)
B=1-2^2018
S=3/2.1-2^2018=3/2^2018
Tờ làm luôn, ko ghi đề nữa nhé
\(A=\frac{\frac{24}{12}-\frac{4}{12}+\frac{3}{12}}{\frac{24}{12}+\frac{2}{12}-\frac{3}{12}}\)
\(A=\frac{\frac{23}{12}}{\frac{23}{12}}=1\)
Vậy A=1
\(A=\frac{2-\frac{1}{3}+\frac{1}{4}}{2+\frac{1}{6}-\frac{1}{4}}\)\(=\frac{2-\frac{2}{6}+\frac{2}{8}}{2+\frac{2}{12}-\frac{2}{8}}\)\(=\frac{2\left(1-\frac{1}{6}+\frac{1}{8}\right)}{-2\left(1-\frac{1}{12}+\frac{1}{8}\right)}\)\(=-1\)
A = 1/2^2 + 1/3^2 + 1/4^2 + ... + 1/100^2
1/2^2 < 1/1*2
1/3^2 < 1/2*3
1/4^2 < 1/3*4
...
1/100^2 < 1/99*100
=> A < 1/1*2 + 1/2*3 + 1/3*4 + ... + 1/99*100
=> A < 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/99 - 1/100
=> A < 1 - 1/100
=> A < 1
minh deo can ban k dau :((
\(a,\frac{1}{2}x+\frac{3}{5}(x-2)=3\)
\(\Rightarrow\frac{1}{2}x+\frac{3}{5}x-\frac{6}{5}=3\)
\(\Rightarrow\left[\frac{1}{2}+\frac{3}{5}\right]x=3+\frac{6}{5}\)
\(\Rightarrow\left[\frac{5}{10}+\frac{6}{10}\right]x=\frac{21}{5}\)
\(\Rightarrow\frac{11}{10}x=\frac{21}{5}\)
\(\Rightarrow x=\frac{21}{5}:\frac{11}{10}=\frac{21}{5}\cdot\frac{10}{11}=\frac{21}{1}\cdot\frac{2}{11}=\frac{42}{11}\)
Vậy x = 42/11