(3x+4y)²-(3x-4y)²
=(3x+4y+3x-4y)(3x+4y-3x+4y)
=(6x)(8y)
=24xy
或(3x+4y)²-(3x-4y)²
=(9x²+12xy+16y²)-(9x²+12xy+16y²)
=9x²+12xy+16y²-9x²+12xy-16y²
=24xy