2
a
\(\left|2x+7\right|+\left|2x-1\right|=\left|2x+7\right|+\left|1-2x\right|\ge\left|2x+7+1-2x\right|=8\)
Dấu "=" xảy ra tại \(-\frac{7}{2}\le x\le\frac{1}{2}\)
3
\(3a^2+4b^2=7ab\)
\(\Leftrightarrow3a^2-7ab+4b^2=0\)
\(\Leftrightarrow\left(3a^2-3ab\right)+\left(4b^2-4ab\right)=0\)
\(\Leftrightarrow3a\left(a-b\right)-4b\left(a-b\right)=0\)
\(\Leftrightarrow\left(3a-4b\right)\left(a-b\right)=0\)
\(\Leftrightarrow\orbr{\begin{cases}a=b\\3a=4b\end{cases}}\)
Làm nốt