| acosA=bcosB a・{(b^2+c^2−a^2)/2bc}=b・{(c^2+a^2−b^2)/2ca} a・{(b^2+c^2−a^2)/b}=b・{(c^2+a^2−b^2)/a} a^2・(b^2+c^2−a^2)=b^2・(c^2+a^2−b^2) a^2・b^2+c^2・a^2−a^4=b^2・c^2+a^2・b^2−b^4 c^2・a^2−a^4=b^2・c^2−b^4 c^2・a^2−b^2・c^2−a^4+b^4=0 c^2(a^2−b^2)−(a^4−b^4)=0 c^2(a^2−b^2)−(a^2+b^2)(a^2−b^2)=0 (c^2−a^2−b^2)(a^2−b^2)=0 (c^2−a^2−b^2)(a−b)(a+b)=0 a+b≠0 より, (c^2−a^2−b^2)(a−b)=0 よって,, c^2−a^2−b^2=0,a−b=0 ∴ c^2=a^2+b^2,a=b
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