`mathcal{Rùa *** }`

Ta có : tổng `3` góc `1 Delta = 180^o`

=> Xét `ΔABC` có : 

`hat{A} + hat{B} + hat{C} = 180^o`.

=> `hat{A} + 1/2 hat{C} + hat{C} = 180^o`. ( `hat{B} =1/2 hat{C}`).

=> `1/2 hat{C} + hat{C} = 180^o - hat{A}`

 => `(1/2 +1 )hat{C}     = 180^o -60^o`

=> `3/2 hat{C}         = 120^o`

=> `hat{C}              = 120^o : 3/2 = 80^o`

=> `hat {B} = 80^o xx 1/2 = 40^o`

=> Chọn `C . hat{B } =40^o . hat{C } =80^o`