m＞0 n＜0时,
m\lml+n\lnl=1-1=0
m＜0 n＞0时,
m\lml+n\lnl=-1+1=0
m＜0 n＜0时,
m\lml+n\lnl=-1-1=-2
m＞0 n＞0时,
m\lml+n\lnl=1+1=2