|PA|²=(3cosx-a)^2+4sin²x=9cos²x-6acosx+a²+4(1-cos²x)=5cos²x-6acosx+a²+4再令t=cosx, 则|PA|²=5t²-6at+a²+4=5(t-3a/5)²-4a²/5+4若0<3a/5<=1,即0若3a/5>1, 即5/3综合得：a=2
