等差幂线\(AB\perpCD\iffAC^2-AD^2=BC^2-BD^2\)圆幂定义一个点关于\(\odotO\)的圆幂\(\rho_o(A)=OA^2-R^2\)：若\(A\)在圆外，\(APQ\)是\(\odotO\)的割线，则\(AP\cdotAQ=\rho_o(A)\)若\(A\)在圆内，\(PAQ\)是\(\odotO\)的弦，则\(AP\cdotAQ=-\rho_o(A