y²=2px,焦点F(p/2,0)
设过F的参数方程为
x=p/2+tcosθ
y=tsinθ
θ为直线倾角,t为直线上一点与F的距离,t>0,点在F上方,t<0,点在F下方
设直线与抛物线的交点A、B,A在上方,对应t1,t2(t2<0)
面积=S△AOF+S△BOF
=(1/2)OF.AFsinθ+(1/2)OF.BF.sinθ
=(1/2)(p/2)sinθ(t1-t2)
=(p/4)(t1-t2)sinθ
周长:
AB=t1-t2,
余弦定理:
AO=√(OF²+AF²+2OF.AFcosθ)
=√(p²/4+t1²+pt1cosθ)
BO=√(p²/4+t2²-pt2cosθ)
周长=t1-t2+√(p²/4+t1²+pt1cosθ)+√(p²/4+t2²-pt2cosθ)
x,y代入
抛物线方程得:
t²sin²θ=2p(p/2+tcosθ)
t²sin²θ-2ptcosθ-p²=0
解此方程,求出t1,t2,或者根据
韦达定理t1+t2=2pcosθ/sin²θ
t1t2=-p²/sin²θ
t1-t2=√(t1-t2)²=√[(t1+t2)²-4t1t2]
=√[4p²cos²θ/sin^(4)θ+4p²/sin²θ]
=2p/sinθ√(cos²θ/sin²θ+1)
=2p/sinθ√(cot²θ+1)
=2p/sinθ×cscθ
=2p/sin²θ
面积=(p/4)(t1-t2)sinθ=(p/4)(2p/sin²θ)sinθ=p²/2sinθ
周长=t1-t2+√(p²/4+t1²+pt1cosθ)+√(p²/4+t2²-pt2cosθ)
=2p/sin²θ+√[p²/4+t1²+pt1cosθ+p²/4+t2²-pt2cosθ+2√[(p²/4+t1²+pt1cosθ)(p²/4+t2²-pt2cosθ)]]
=2p/sin²θ+√[p²/2+(t1²+t2²)+pcosθ(t1-t2)+2√[(p²/4+t1²+pt1cosθ)(p²/4+t2²-pt2cosθ)]]