Tính nhanh:
\(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{40.43}\)
\(=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{40}-\frac{1}{43}\)
\(=1-\frac{1}{43}\)
\(=\frac{42}{43}\)
Tìm x:
a) \(x+30\%x=-1,3\)
\(\Leftrightarrow x\left(1+30\%\right)=-1,3\)
\(\Leftrightarrow x\left(1+\frac{3}{10}\right)=\frac{-13}{10}\)
\(\Leftrightarrow x.\frac{13}{10}=\frac{-13}{10}\)
\(\Leftrightarrow x=\frac{-13}{10}:\frac{13}{10}=\frac{-13}{10}.\frac{10}{13}=-1\)
Vậy \(x=-1\)
b) \(\left|2x-1\right|=\left(-4\right)^2\)
\(\Leftrightarrow\left|2x-1\right|=16\)
\(\Rightarrow\left[{}\begin{matrix}2x-1=16\\2x-1=-16\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\frac{17}{2}\\x=\frac{-15}{2}\end{matrix}\right.\)
Vậy \(x\in\left\{\frac{17}{2};\frac{-15}{2}\right\}\)
