A)  7/38 x 9/11 +7/38 x 4/11 -7/38 x 2/11

=7/38.(9/11+4/11-2/11)

=7/38

B) 5/31 x 21/25 + 5/31 x -7/10 - 5/31 x 9/20

=5/31.(21/25-7/10-9/20)

=5/31.(-31/100)

=-1/20

à mà có phải í bn là: x/17 x (1,2 + 1/3) = 31/51

Đáp án B.

1.

b) \(3^x+3^{x+2}=2430\)

\(\Rightarrow3^x.1+3^x.3^2=2430\)

\(\Rightarrow3^x.\left(1+3^2\right)=2430\)

\(\Rightarrow3^x.10=2430\)

\(\Rightarrow3^x=2430:10\)

\(\Rightarrow3^x=243\)

\(\Rightarrow3^x=3^5\)

\(\Rightarrow x=5\)

Vậy \(x=5.\)

c) \(\left(2x-15\right)^5=\left(2x-15\right)^3\)

\(\Rightarrow\left(2x-15\right)^5-\left(2x-15\right)^3=0\)

\(\Rightarrow\left(2x-15\right)^3.\left[\left(2x-15\right)^2-1\right]=0\)

\(\Rightarrow\left[{}\begin{matrix}\left(2x-15\right)^3=0\\\left(2x-15\right)^2-1=0\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}2x-15=0\\\left(2x-15\right)^2=1\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}2x=15\\2x-15=\pm1\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=15:2\\2x-15=1\\2x-15=-1\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=\frac{15}{2}\\2x=16\\2x=14\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=\frac{15}{2}\\x=8\\x=7\end{matrix}\right.\)

Vậy \(x\in\left\{\frac{15}{2};8;7\right\}.\)

Chúc bạn học tốt!

\(\frac{x}{\frac{1}{2}}+\frac{x}{\frac{1}{3}}+x=42\)

\(\Rightarrow\frac{x}{\frac{3}{6}}+\frac{x}{\frac{2}{6}}+\frac{x}{\frac{6}{6}}=42\)

\(\Rightarrow\frac{x}{\frac{3}{6}+\frac{2}{6}+\frac{6}{6}}=42\)

\(\Rightarrow\frac{x}{\frac{11}{6}}=42\)

\(\Rightarrow x=77\)

\(x.\frac{4}{7}+x.\frac{2}{3}+x.\frac{3}{7}=\frac{11}{5}\)

\(\Rightarrow x.\left(\frac{4}{7}+\frac{3}{7}+\frac{2}{3}\right)=\frac{11}{5}\)

\(\Rightarrow x.\frac{5}{3}=\frac{11}{5}\)

\(\Rightarrow x=\frac{33}{25}\)

\(\left(\frac{31}{5}-x\right):\frac{2}{3}=\frac{7}{6}\)

\(\Rightarrow\frac{31}{5}-x=\frac{7}{9}\)

\(\Rightarrow x=\frac{244}{45}\)

\(\frac{4}{3}+\left(x-\frac{3}{5}\right)=\frac{21}{2}\)

\(\Rightarrow\frac{4}{3}+x-\frac{3}{5}=\frac{21}{2}\)

\(\Rightarrow x+\frac{11}{5}=\frac{21}{2}\)

\(\Rightarrow x=\frac{83}{10}=8,3\)