后面的每一个项都可以裂成两项的差。1/(x+10)+1/(x+1)(x+2)+…+1/(x-9)(x+10)=2/5
1/(x+10)+1/(x+1)-1/(x+2)+....+1/(x+9)-1/(x+10)=2/5
1/(x+1)=2/5
x+1=5/2
x=3/2
1/(x+10)+1/(x+1)(x+2)+…+1/(x-9)(x+10)=2/5
1/(x+10)+1/(x+1)-1/(x+2)+1/(x+2)-1/(x+3)+1/(x+3)-1/(x+4)+.......+1/(x+9)-1/(x+10)=2/5
1/(x+1)=2/5
2(x+1)=5
2x+2=5
2x=3
x=3/2
1/(x-9)(x+10)?
其他项的正负是什么规律?