1+2-3-4+5+6-7-8+.-1983-1984+1985+1986
=(1+2-3-4)+(5+6-7-8)+.+(1981+1982-1983+1984)+1985+1986
=-4+(-4)+.+(-4)+1985+1986
=-4*496+1985+1986=1987
这里是加1/42吧,如果是,就为
1-1/2+1/6+1/12+1/20+1/30+1/40+1/72+1/56+1/90
=1-1/2+(1/2-1/3)+(1/3-1/4)+.+(1/9-1/10)
=1-1/10=0.9
如果不是,把答案改改就可以了