Câu hỏi:

Cho ΔABC  thỏa mãn  c = 2bcosA và . Tìm mệnh đề đúng?

A. tam giác cân tại C

B. tam giác cân tại B

C. tam giác đều

Đáp án chính xác

D. tất cả sai

Trả lời:

Chọn C.
Ta có: 
⇔ c3 + a3 – b3 = b2c + ab2 – b3
⇔ c3 + a3 = b2c + ab2
⇔ (c + a)(c2 – ca + a2) = b2(c + a)
⇔ c2 – ca + a2 = b2
⇔ c2 – ca + a2 = a2 + c2 – 2accosB

Mà: c = 2b cosA 
Ta lại có a2 = b2 + c2 – 2bc cosA
⇔ a2 = b2 + c2 – 2bc. c/2b ⇔ a2 = b2 ⇔ a = b
Vậy ΔABC đều.

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