With usual notations

MATHEMATICS · Subjective · A Level · Mathematics · Trigonometry · Trigonometry

Problem

With usual notations, if in a △ABC\triangle ABC, b+c11=c+a12=a+b13\frac{b+c}{11}=\frac{c+a}{12}=\frac{a+b}{13}, prove that cos⁡A7=cos⁡B19=cos⁡C25.\frac{\cos A}{7}=\frac{\cos B}{19}=\frac{\cos C}{25}.

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