（2)(x-2)(x+2)-(x+1)(x-3)
=x²-4-(x²-2x-3)
=x²-4-x²+2x+3
=2x-1
(3) (ab+1)²-(ab-1)²
=(ab+1+ab-1)(ab+1-ab+1)
=2(2ab)
=4ab
(4)（2x-y)²－4(x-y)(x+2y)
=4x²-4xy+y²-4(x²+xy-2y²)
=4x²-4xy+y²-4x²-4xy+8y²
=-8xy+9y²