\(3+\left(x\cdot4\right)-14=1\Leftrightarrow4x=12\Leftrightarrow x=3\)

\(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+....+\frac{1}{49^2}+\frac{1}{50^2}\) 

< \(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+....+\frac{1}{48.49}+\frac{1}{49.50}\)

j) \(x^2+y^2+26+10x-2y=0\)

\(\Leftrightarrow\left(x^2+10x+25\right)+\left(y^2-2y+1\right)=0\)

\(\Leftrightarrow\left(x+5\right)^2+\left(y-1\right)^2=0\)

\(\Leftrightarrow\begin{cases}x+5=0\\y-1=0\end{cases}\)\(\Leftrightarrow\begin{cases}x=-5\\y=1\end{cases}\)

Vậy x=-5; y=1

h) \(4x^2-4x+1=0\)

\(\Leftrightarrow\left(2x-1\right)^2=0\)

\(\Leftrightarrow2x-1=0\)

\(\Leftrightarrow x=\frac{1}{2}\)

\(\left(x^2-4\right)+\left(x-2\right)^2=0\)

\(\Leftrightarrow\left(x-2\right)\left(x+2\right)+\left(x-2\right)^2=0\)

\(\Leftrightarrow\left(x-2\right)\left(x+2+x-2\right)=0\)

\(\Leftrightarrow2x\left(x-2\right)=0\)

\(\Leftrightarrow\left[\begin{array}{nghiempt}x=0\\x-2=0\end{array}\right.\)\(\Leftrightarrow\left[\begin{array}{nghiempt}x=0\\x=2\end{array}\right.\)