\(\Delta ABC\) : vuông tại B
\(\Rightarrow\widehat{ABC}=90^o;Mà:\widehat{BAC}=40^o\\ Tổng.số.đo.các.góc.trong.1.tam.giác.bằng.180^o:\widehat{ABC}+\widehat{BAC}+\widehat{ACB}=180^o\\ \Leftrightarrow90^o+40^o+\widehat{ACB}=180^o\\ \Leftrightarrow\widehat{ACB}=180^o-\left(90^o+40^o\right)=50^o\)
b, Vì: 40o < 50o < 90o => BC < AB < AC