∠APB=θ\angle APB=\theta∠APB=θ とおくと,余弦定理より, cosθ=AP2+BP2−c22AP⋅BP\cos\theta=\dfrac{AP^2+BP^2-c^2}{2AP\cdot BP}cosθ=2AP⋅BPAP2+BP2−c2 , cos(180∘−θ)=AP2+CP2−b22AP⋅CP\cos(180^{\circ}-\theta)=\dfrac{AP^2+CP^2-b^2}{2AP\cdot CP}cos(180∘−θ)=2AP⋅CPAP2+CP2−b2 これらと cosθ=−cos(180∘−θ)\cos\theta=-\cos(180^{\circ}-\theta)cosθ=−cos(180∘−θ) より, CP(AP2+BP2−c2)=−BP(AP2+CP2−b2)CP(AP^2+BP^2-c^2)=-BP(AP^2+CP^2-b^2
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