\(7x^2+2x+1=0\)
=>\(7\left(x^2+\dfrac{2}{7}x+\dfrac{1}{7}\right)=0\)
=>\(x^2+\dfrac{2}{7}x+\dfrac{1}{7}=0\)
=>\(x^2+2\cdot x\cdot\dfrac{1}{7}+\dfrac{1}{49}+\dfrac{6}{49}=0\)
=>\(\left(x+\dfrac{1}{7}\right)^2+\dfrac{6}{49}=0\)(vô lý)
=>\(x\in\varnothing\)