25^x=2000, 得：x=log25(2000)=lg2000/lg25=3lg2/lg2580^y=2000, 得：y=log80(2000)=lg2000/lg80=3lg2/lg80x+y-xy=3lg2/lg25+3lg2/lg80-3lg2/lg25*3lg2/lg80=3lg2(lg80+lg25-1)/lg25lg80=3lg2(3lg2-1)/lg25lg80
