a) Gọi ABC là tam giác cân đã cho và góc ở đỉnh \(\widehat{A}\) = 40⁰

Ta có: \(\widehat{A}\) + \(\widehat{B}\) + \(\widehat{C}\) = 180⁰

mà  \(\widehat{B}\) = \(\widehat{C}\)

=>2\(\widehat{B}\)=  180⁰ -  \(\widehat{A}\) = 140⁰

=> \(\widehat{B}\)=  70⁰

b) Ta có: \(\widehat{A}\) + \(\widehat{B}\) + \(\widehat{C}\) = 180⁰

mà  \(\widehat{B}\) = \(\widehat{C}\) = 40⁰

=> \(\widehat{A}\) + 2\(\widehat{B}\) = 180⁰

=>\(\widehat{A}\) + 80⁰ = 180⁰

 \(\widehat{A}\)=40⁰