\(A=x^2+x+1\)
\(A=x^2+x+\dfrac{1}{4}-\dfrac{1}{4}+1\)
\(A=\left(x+\dfrac{1}{2}\right)^2+\dfrac{3}{4}\)
mà \(\left(x+\dfrac{1}{2}\right)^2\ge0\)
\(\Rightarrow A=\left(x+\dfrac{1}{2}\right)^2+\dfrac{3}{4}>\dfrac{3}{4}>0\) với mọi x
\(\Rightarrow Dpcm\)