必须满足k≠0且判别式△＞0,即：
（k+1）²-4×k×k/4＞0
解得：k＞-1/2且k≠0
x1+x2=-(k+1)/k=-1-1/k
x1x2=(k/4)/k=1/4
1/x1+1/x2=(x1+x2)/(x1x2)=-(1+1/k)/(1/4)=0
1+1/k=0
k=-1