x^2/4+y^2=1上的任意点的坐标是x=2cost,y=sint
1)设S=(y+4)/x=(sint+4)/(2cost)
--->2Scost=sint+4
--->sint-2Scost=-4
两边同时除√(1+4S^2)得
sin(t-f)=-4/√(1+4S^2) tant=2S
|sin(t-f)|=4/√(1+4S^2)=√(1+4S^2)>=4
--->1+4S^2>=16
--->S^2>=15/4
--->S>=√15/2或者S=