Answer
$$(2x+1)^2 .$$
Work Step by Step
The polynomial $ 4x^2+4x+1 $ is a complete square since it is of the form $a^2+2ab+b^2=(a+b)^2$. Thus, by putting $a=2x$, $b=1$, we have
$$ 4x^2+4x+1=(2x+1)^2 .$$
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