(t²-1)/(1+t)²=(t+1)(t-1)/(t+1)²=(t-1)/(t+1)=(t+1-2)/(t+1)=1-2/(t+1)
当t>-1时,2/(t+1)>0.随t减小,1+t减小,2/(t+1)增大,1-2/(t+1)减小.
当t->-1时,1-2/(t+1)->-∞
最小值是不存在的.