若a>b>0,c若a>b>0,c
若a>b>0,ca>b>0-c>-d>0-ca>-db>0两边同除以cd-a/d>-b/c>0a/d
a>b>0,c|a |>|b|,|c|>|d|=>|ac |>|bd|,ac<0,bc<0=>acac/dca/d