27,72 : 3 + x × 6 = 28,44
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\(4,75+x:9,4=34,266\)
=>\(x:9,4=34,266-4,75=29,516\)
=>\(x=29,516\cdot9,4=277,4504\)

Ta có: \(\frac{x}{20}+\frac{x}{30}+\frac{x}{42}=3\)
=>\(x\left(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\right)=3\)
=>\(x\left(\frac14-\frac15+\frac15-\frac16+\frac16-\frac17\right)=3\)
=>\(x\left(\frac14-\frac17\right)=3\)
=>\(x\cdot\frac{3}{21}=3\)
=>\(x=3:\frac{3}{21}=21\)


\(\frac{2}{1\cdot2}+\frac{2}{2\cdot3}+\cdots+\frac{2}{19\cdot20}\)
\(=2\left(\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\cdots+\frac{1}{19\cdot20}\right)\)
\(=2\left(1-\frac12+\frac12-\frac13+\cdots+\frac{1}{19}-\frac{1}{20}\right)\)
\(=2\left(1-\frac{1}{20}\right)=2\cdot\frac{19}{20}=\frac{19}{10}\)
Đặt \(A=\frac{2}{1\times2}+\frac{2}{2\times3}+\cdots+\frac{2}{18\times19}+\frac{2}{19\times20}\)
Ta có:
\(A=2\times\left(\frac{1}{1\times2}+\frac{1}{2\times3}+\cdots+\frac{1}{18\times19}+\frac{1}{19\times20}\right)\)
\(A=2\times\left(\frac{2-1}{1\times2}+\frac{3-2}{2\times3}+\cdots+\frac{19-18}{18\times19}+\frac{20-19}{19\times20}\right)\)
\(A=2\times\left(\frac{2}{1\times2}-\frac{1}{1\times2}+\frac{3}{2\times3}-\frac{2}{2\times3}+\cdots+\frac{20}{19\times20}-\frac{19}{19\times20}\right)\)
\(A=2\times\left(1-\frac12+\frac12-\frac13+\ldots+\frac{1}{19}-\frac{1}{20}\right)\)
\(A=2\times\left(1-\frac{1}{20}\right)\)
\(A=2\times\frac{19}{20}\)
\(A=\frac{19}{10}\)
Vậy \(A=\frac{19}{10}\)

Đặt \(A=2^2+2^3+\cdots+2^{2005}\)
=>\(2A=2^3+2^4+\cdots+2^{2006}\)
=>\(2A-A=2^3+2^4+\cdots+2^{2006}-2^2-2^3-\cdots-2^{2005}\)
=>\(A=2^{2006}-4\)
\(C=4+2^2+2^3+\cdots+2^{2005}\)
\(=4+2^{2006}-4=2^{2006}\)
=>C là lũy thừa có cơ số 2


\(2^{100}\) và \(1024^9\)
\(1024^9=\left(2^{10}\right)^9=2^{90}<2^{100}\)
Vậy \(2^{100}>1024^9\)
\(27,72:3+x\times6=28,44\)
\(9,24+x\times6=28,44\)
\(x\times6=28,44-9,24\)
\(x\times6=19,2\)
\(x=19,2:6=3,2\)
9,24 + x . 6 = 28,44
x . 6 = 28,44 - 9,24
x . 6 = 19,20
x = 19,20 : 6
x = 3,20
Vậy x=3,20