解
(x+2y)²-(x-2y+3)(x-2y-3)
=(x+2y)²-[(x-2y)+3][(x-2y)-3]
=x²+4xy+4y²-[(x-2y)²-9]
=x²+4xy+4y²-[x²-4xy+4y²-9]
=8xy+9
=8×(27/4)×(-1/9)+9
=-6+9
=3