b2x2+a2y2=a2b2x2/a2+y2/b2=1e2=a2/(a2+b2)=a2>0,b2>0,a20,由P点向x轴引垂线,垂足为M,则∠APM+∠BPM=90o∴tan120o=tan(∠APM+∠BPM)===>-√3=[(a+x+a-x)/y]/[1-(a2-x2)/y2]=2ay/[y-a2+x2],由P点在椭圆上,消去x2,得:y=2ab2/[√3c2]∵00<2ab/[√3(a2-b2)]≤1,√3a2-2ab-√3b2≥0解得:a≥√3b,或a≤-b/√3(舍去)∴b2/a2≤1/3,由e2=1-b2/a2,得:e2≥2/3,∴e≥√6/3又:0<e<1∴√6/3<e<1(a^2 -c^2)*(c-a)^2 =c^2*(2a^2 -c^2)(1-e^2)(1-1/e)^2=(2-e^2)(1-e^2)(-2/e+1/e^2)=1(1-e^2)(-2e+1)-e^2=02e^3-2e^2-2e+1=0
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