(x-3y)(x+3y)-(x-3y)^2
=x²-9y²-(x²-6xy+9y²)
=-18y²+6xy
112^2-113×111
=112²-(112+1)(112-1)
=112²-(112²-1²)
=1
（2x-3）（x+4）
=2x²+8x-3x-12
=2x²+5x-12