x=1/x
则x²=1
x=±1
A=(x+3)²/(x-3)
B=(x+3)/(x-3)²
所以M=AB
=(x+3)³/(x-3)³
=(-1+3)³/(-1-3)³=-1/8
或=(1+3)³/(1-3)³=-8
N=a÷B
=(x+3)(x-3)
=x²-9
=1-9
=-8