(2m+n)^2-(m-n)^2

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(2m+n)^2-(m-n)^2
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(2m+n)^2-(m-n)^2
(2m+n)^2-(m-n)^2

(2m+n)^2-(m-n)^2
(2m+n)^2-(m-n)^2
=4m²+4mn+n²-(m²-2mn+n²)
=3m²+6mn

=[(2m+n)+(m-n)][(2m+n)-(m-n)]
=3m(m+2n)

4m^2+4mn+n^2-m^2+2mn-n^2=3m^2+2mn