dragonlincer
花朵
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设MN的坐标分别为(Xm,Ym),(Xn,Yn),则直线OM,ON的斜率分别为Ym/Xm,Yn/Xn.
所以Ym/Xm*Yn/Xn=-b^2/a^2,Xm^2/a^2+Ym^2/b^2=1,Xn^2/a^2+Yn^2/b^2=1.
所以(Ym^2*Yn^2)/(Xm^2*Xn^2)=b^4/a^4 (1)
Xm^2=a^2*(1-Ym^2/b^2) (2);Xn^2=a^2*(1-Yn^2/b^2) (3)
(2)(3)带入(1)可得:Ym^2+Yn^2=b^2 (4)
所以Xm^2+Xn^2=a^2*(1-Ym^2/b^2)+a^2*(1-Yn^2/b^2)=a^2*(2-Ym^2/b^2-Yn^2/b^2)=a^2
|OM|^2+|ON|^2=(Xm^2+Ym^2)+(Xn^2+Yn^2)=(Xm^2+Xn^2)+(Ym^2+Yn^2)=a^2+b^2
1年前
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