巧求和：1000+999-998+997+996-995+...+106+105-104+103+102-101=

x=1000+999-998+997+996-995+...+106+105-104+103+102-101
=[999+1+999-(999-1)]+(996+1+996-(996-1)]+...+[105+1+105-(105-1)]+[102+1+102-(102-1)]
=(999+2)+(996+2)+...+(105+2)+(102+2)
=(102+105+....+999)+