\(=\left(3x+1-x-1\right)\left(3x+1+x+1\right)=2x\left(4x+2\right)=4x\left(2x+1\right)\)
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\(\left(3x+1\right)^2-\left(x+1\right)^2=\left(3x+1-x-1\right)\left(3x+1+x+1\right)=2x\left(4x+2\right)\)
\(=2x\cdot2\left(2x+1\right)=4x\cdot\left(2x+1\right)\)
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(3x+1)2−(x+1)2=(3x+1−x−1)(3x+1+x+1)=2x(4x+2)(3x+1)2−(x+1)2=(3x+1−x−1)(3x+1+x+1)=2x(4x+2)
=2x⋅2(2x+1)=4x⋅(2x+1)
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