Cho tam giác $ ABC $ , biết: $ \widehat{A}={{70}^{0}};\,\,\widehat{B}-\widehat{C}={{10}^{0}}. $ Khi đó: A. $ \widehat{B}={{60}^{0}} $ . B. $ \widehat{B}={{50}^{0}}. $ C. $ \widehat{B}={{80}^{0}}. $ D. $ \widehat{B}={{40}^{0}}. $
Đáp án đúng: A Ta có: $ \widehat{A}+\widehat{B}+\widehat{C}={{180}^{0}};\,\,\widehat{A}={{70}^{0}}\Rightarrow \widehat{B}+\widehat{C}={{110}^{0}} $ . Mà $ \widehat{B}-\widehat{C}={{10}^{0}}\Rightarrow \widehat{B}={{60}^{0}};\,\,\widehat{C}={{50}^{0}}. $