Ta có: \(125,5:5+x=37,86\)

=>25,1+x=37,86

=>x=37,86-25,1=12,76

25,1 + x = 37,86

x = 37,86 - 25,1

x = 12,76

Vậy x=12,76

\(27,72:3+x\times6=28,44\)

\(9,24+x\times6=28,44\)

\(x\times6=28,44-9,24\)

\(x\times6=19,2\)

\(x=19,2:6=3,2\)

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

\(19,5-x\times6,3=11,625\)

\(x\times6,3=19,5-11,625\)

\(x\times6,3=7,875\)

\(x=7,875:6,3=1,25\)

x . 6,3 = 19,5 - 11,625

x . 6,3 = 7,875

x = 7,875 : 6,3

x =1,25

Vậy x=1,25

\(4,75+x:9,4=34,266\)

=>\(x:9,4=34,266-4,75=29,516\)

=>\(x=29,516\cdot9,4=277,4504\)

x : 9,4 = 34,266 - 4,75

x : 9,4 = 29,516

x = 29,516 . 9,4

x = 277,4504

Vậy x=277,4504

Ta có: \(\frac{x}{20}+\frac{x}{30}+\frac{x}{42}=3\)

=>\(x\left(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\right)=3\)

=>\(x\left(\frac14-\frac15+\frac15-\frac16+\frac16-\frac17\right)=3\)

=>\(x\left(\frac14-\frac17\right)=3\)

=>\(x\cdot\frac{3}{21}=3\)

=>\(x=3:\frac{3}{21}=21\)

\(\frac{x}{20}+\frac{x}{30}+\frac{x}{42}=\) \(3\)

\(\frac{21x}{420}+\frac{14x}{420}+\frac{10x}{420}=3\)

\(\frac{45x}{420}=3\)

\(45x=3\times420\)

\(45x=1260\)

\(x=1260:45\)

\(x=28\)

Đáp án:Ta có 

11.3^29-3^30/2^2.3^28

=3^29.(11-3)/4.3^28

=3^29.8/4.3^28

=6.

Nhớ tick mình nhé, chúc bạn học tốt!

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 \(A=\frac{19}{10}\)

\(\frac{2}{1\cdot2}+\frac{2}{2\cdot3}+\cdots+\frac{2}{19\cdot20}\)

\(=2\left(\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\cdots+\frac{1}{19\cdot20}\right)\)

\(=2\left(1-\frac12+\frac12-\frac13+\cdots+\frac{1}{19}-\frac{1}{20}\right)\)

\(=2\left(1-\frac{1}{20}\right)=2\cdot\frac{19}{20}=\frac{19}{10}\)

Đặt \(A=2^2+2^3+\cdots+2^{2005}\)

=>\(2A=2^3+2^4+\cdots+2^{2006}\)

=>\(2A-A=2^3+2^4+\cdots+2^{2006}-2^2-2^3-\cdots-2^{2005}\)

=>\(A=2^{2006}-4\)

\(C=4+2^2+2^3+\cdots+2^{2005}\)

\(=4+2^{2006}-4=2^{2006}\)

=>C là lũy thừa có cơ số 2

help me pls

\(\overline{3}^{x^{}}=81\Rightarrow x=4\)