解:1/1990*1992+1/1992*1994+1/1994*1996+1/1996*1998+1/1998*2000 =1/2(1/1990 - 1/1992)+ 1/2(1/1992 - 1/1994)+...+ 1/2(1/1998 - 1/2000) =1/2(1/1990 - 1/1992 + 1/1992 - 1/1994 +...+ 1/1998 - 1/2000) =1/2(1/1990 - 1/2000)= 5/(1990*2000)= 1/796000