(a-2b+c+3d)(a+2b-c+3d)
=[(a+3d)+(c-2b)][(a+3d)-(c-2b)]
=(a+3d)²-(c-2b)²
=a²+6ad+9d²-c²+4bc-4b²
(a-2b+c+3d)(a+2b-c+3d)
=[(a+3d)+(c-2b)][(a+3d)-(c-2b)]
=(a+3d)²-(c-2b)²
=a²+6ad+9d²-c²+4bc-4b²
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