1、2(x+1/x)²-4-3(x+1/x)-1=0
2(x+1/x)²-3(x+1/x)-5=0
11
2-5
所以x+1/x=-1或x+1/x=5/2
解得x=2或1/2
2、[(1-1/3)+(1/2-1/4)+(1/3-1/5)+...+(1/9-1/11)]/2=(1+1/2-1/10-1/11)/2=36/55
3、1/[n(n+1)(n+2)]={1/[n(n+1)]-1/[(n+1)(n+2)]}/2
所以左边={[1/(1*2)-1/(2*3)]+[1/(2*3)-1/(3*4)]+...+1/[n(n+1)]-1/[(n+1)(n+2)]}/2
={1/2-1/[(n+1)(n+2)]}/2
<1/4.