\(\text{Áp dụng HĐT }A^2-B^2\)
\(\left(3x+1\right)^2-\left(x+1\right)^2\)
\(=\left(3x+1-x+1\right)\left(3x+1+x+1\right)\)
\(=\left(2x+2\right)\left(4x+2\right)\)
(3x+1)2-(x+1)2
=(3x)2+6x+1-(x2+2x+1)
=9x2+6x+1-x2-2x-1
=9x2-x2+6x-2x+1-1
=8x2+4x
=4x(2x+1)