1-2+3-4+5-6+7-.+1989-1990+1991=?
1-2+3-4+5-6+7-.+1989-1990+1991=?
1+2+3+4-5-6-7-8+...-1989-1990-1991-1992+1993+1994+1995+1996=
1+2-3-4+5+6-7-8+9+10-11-12…+1989+1990-1991-1992+1993+1994=__
1+2+3-4+5-6+……1989-1990+1991
1-2+3-4+5-6+.+1989-1990+1991简便计算
1-2+3-4+5-6……+1989-1990+1991怎样简便算?
1-2+3-4+5-6……+1989-1990+1991
简算:1-2+3-4+5-6…+1989-1990+1991
1-2+3-4+5-6+7-8...-1990+1991
1994-1993+1992+1991-1990+1989+1988-1987+.+5-4+3+2-1=
1+3+5+7+``````+1989=?2+4+6+8+````+1990=?
(2+4+6+8+...+1990)—(1+3+5+7+.+1989)=?