2.5^x=1000x=log2.5(1000)=3log2.5(10)=3/lg2.50.25^y=1000y=log0.25(1000)=3log0.25(10)=3/lg0.25∴1/x-1/y=(lg2.5)/3-(lg0.25)/3=[lg(2.5÷0.25)]/3=(lg10)/3=1/3