Đề bài

Cho \(\alpha \) thoả mãn \(\sin \alpha  = \frac{3}{5}\). Tính cos\(\alpha \), tan\(\alpha \), cot\(\alpha \), sin(90° - \(\alpha \)), cos(90° - \(\alpha \)), sin(180° – \(\alpha \)), cos(180° – \(\alpha \)) trong các trường hợp sau: a) 0° < \(\alpha \) < 90° b) 90° < \(\alpha \) < 180°

Lời giải chi tiết

a) Theo giả thiết, 0° < \(\alpha \) < 90° \( \Rightarrow \cos \alpha  > 0,\tan \alpha  > 0,\cot \alpha  > 0\)+ Ta có: \({\sin ^2}\alpha  + {\cos ^2}\alpha  = 1 \Rightarrow {\cos ^2}\alpha  = 1 - {\sin ^2}\alpha  = 1 - {\left( {\frac{3}{5}} \right)^2} = \frac{{16}}{{25}}\) \( \Rightarrow \cos \alpha  = \frac{4}{5}\)+ \(\tan \alpha  = \frac{{\sin \alpha }}{{\cos \alpha }} = \frac{3}{4};\cot \alpha  = \frac{{\cos \alpha }}{{\sin \alpha }} = \frac{4}{3}\)+ \(\sin ({90^0} - \alpha ) = \cos \alpha  = \frac{4}{5};\cos ({90^0} - \alpha ) = \sin \alpha  = \frac{3}{5}\)+ \(\sin \left( {{{180}^0} - \alpha } \right) = \sin \alpha  = \frac{3}{5};\cos \left( {{{180}^0} - \alpha } \right) =  - \cos \alpha  =  - \frac{4}{5}\)b) Theo giả thiết, 90° < \(\alpha \) < 180° \( \Rightarrow \cos \alpha  < 0,\tan \alpha  < 0,\cot \alpha  < 0\)+ Ta có: \({\sin ^2}\alpha  + {\cos ^2}\alpha  = 1 \Rightarrow {\cos ^2}\alpha  = 1 - {\sin ^2}\alpha  = 1 - {\left( {\frac{3}{5}} \right)^2} = \frac{{16}}{{25}}\) \( \Rightarrow \cos \alpha  =  - \frac{4}{5}\) + \(\tan \alpha  = \frac{{\sin \alpha }}{{\cos \alpha }} =  - \frac{3}{4};\cot \alpha  = \frac{{\cos \alpha }}{{\sin \alpha }} =  - \frac{4}{3}\)+ \(\sin ({90^0} - \alpha ) = \cos \alpha  =  - \frac{4}{5};\cos ({90^0} - \alpha ) = \sin \alpha  = \frac{3}{5}\)+ \(\sin \left( {{{180}^0} - \alpha } \right) = \sin \alpha  = \frac{3}{5};\cos \left( {{{180}^0} - \alpha } \right) =  - \cos \alpha  = \frac{4}{5}\)