Question

CM tam giác ABC cân nếu:

1-cosA=\(\dfrac{a^2}{2bc}\)

Answer 1

\(1-cosA=\dfrac{a^2}{2bc}\Rightarrow2bc-2bc.cosA=a^2\\ \Rightarrow2bc-2bc.cosA=b^2+c^2-2bc.cosA\\ \Rightarrow b^2+c^2-2bc=0\Rightarrow b=c\)

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