a) Nếu \({\pi \over 2} < \alpha < \pi \) thì \(\sinα>0\)
\(\sin \alpha = \sqrt {1 - {{\cos }^2}x} = \sqrt {1 - {2 \over 9}} = {{\sqrt 7 } \over 3}\)
b) Nếu \(\pi < \alpha < {{3\pi } \over 2}\) thì \(\cosα<0\)
\(\cos \alpha = - \sqrt {{1 \over {1 + {{\tan }^2}\alpha }}} = - \sqrt {{1 \over {1 + 8}}} = - {1 \over 3}\)
c) Nếu \({{3\pi } \over 2} < \alpha < 2\pi \) thì \(\tan α<0, \, \cosα>0\)
\(\tan\alpha = {{\sin \alpha } \over {\cos \alpha }} = ( - {2 \over 3}):\sqrt {1 - ({2 \over 3}} {)^2} \)\(= - {{2\sqrt 5 } \over 5}\)
d) Nếu \({\pi \over 2} < \alpha < \pi \) thì \(\cotα<0, \, \sinα>0\)
\(\cot \alpha = \left( { - {1 \over 4}} \right):\sqrt {1 - {{\left( {{1 \over 4}} \right)}^2}} = - {{\sqrt {15} } \over 15}\)
