双曲线的两条渐近线的斜率为b/a或-b/a
过A(a,o)的直线的斜率为b/a或-b/a
直线方程为:y=(b/a)(x-a) 或 y=(-b/a)(x-a)
即:y=(b/a)x-b 或 y=(-b/a)x+b
设点P坐标为(m,n)
OP^2=m^2+n^2
直线OP为:y=(n/m)x
联立{y=(n/m)x 与 y=(b/a)x-b
解得:x=-abm/(an-bm) y=-abn/(an-bm)
联立{y=(n/m)x 与 y=(-b/a)x+b
解得:x=abm/(an+bm) y=abn/(an+bm)
则Q,R坐标分别为:
(-abm/(an-bm),-abn/(an-bm))与:(abm/(an+bm),abn/(an+bm))
OQ=√{[-abm/(an-bm)]^2+[-abn/(an-bm)]^2}
=|ab/(an-bm)|√(m^2+n^2)
OR=√{[abm/(an+bm)]^2+[abn/(an+bm)]^2}
=|ab/(an+bm)|√(m^2+n^2)
OQ·OR=[a^2b^2/|(a^2n^2-b^2m^2)|](m^2+n^2)
∵点P在双曲线x^2/a^2-y^2/b^2=1上
∴m^2/a^2-n^2/b^2=1
b^2m^2-a^2n^2=a^2b^2
则a^2b^2/|(a^2n^2-b^2m^2)|=a^2b^2/|a^2b^2|=1
∴OQ·OR=m^2+n^2
∴OP^2=OQ·OR