设A∈椭圆（x²/a²+y²/b²＝1,a＞b＞0）,AF1＝d,AF2＝f,∠F1AF2＝α,从余弦定理：cosα＝｛d²+f²-4[a²-b²]｝/（2df）,∵d²+f²＝4a²-2dfcosα＝[4b²/（2d...