1D x->∝, sinx+1可能为0,1,2极限不存在,且不为无穷B x->∝, 1/x->0 -1<=sinx<=1 -1/x<=sinx/x<=1/x 极限0C x->∝, x->∝, 12y=arccot(1/(1-x)) 无穷间断点
