\(\left(1-2-3+4\right)+\left(5-6-7+8\right)+...+\left(1993-1994\right)\)
\(=0+0+....+\left(-1\right)\)
\(=\left(-1\right)\)
Ta có 1-2-3+4+5-6-7+8+...+1993-1994
=(1-2-3+4)+(5-6-7+8)+...+(1889-1990-1991+1992)+1993-1994
= 0 + +0 +.....+ 0 + 1993-1994
= -1