Biết sin a = 5/ 13 ; cos b = 3 /5 ( pi/ 2 < a < pi ; 0 < b < pi/ 2 ) . Hãy tính sin ( a + b ) .

14/39

Biết \[\sin a = \frac{5}{{13}};\,\,\cos b = \frac{3}{5}\,\,\left( {\frac{\pi }{2} < a < \pi ;\,\,0 < b < \frac{\pi }{2}} \right)\] . Hãy tính \(\sin \left( {a + b} \right)\).

\[\frac{{56}}{{65}}.\]

\[\frac{{ - 33}}{{65}}.\]

\[0.\]

\[\frac{{63}}{{65}}.\]

Giải thích

Chọn B

Ta có \(\sin \left( {a + b} \right) = \sin a\cos b + \sin b\cos a\) mà

\[\sin a = \frac{5}{{13}};\,\,\left( {\frac{\pi }{2} < a < \pi } \right) \Rightarrow \cos a = - \sqrt {1 - {{\sin }^2}a} = - \sqrt {1 - \frac{{25}}{{169}}} = - \frac{{12}}{{13}}\]

\[\,\cos b = \frac{3}{5}\,\,;\left( {0 < b < \frac{\pi }{2}} \right) \Rightarrow \sin b = \sqrt {1 - {{\cos }^2}b} = \sqrt {1 - \frac{9}{{25}}} = \frac{4}{5}\]

Do đó \(\sin \left( {a + b} \right) = \sin a\cos b + \sin b\cos a = \frac{5}{{13}}.\frac{3}{5} - \frac{{12}}{{13}}.\frac{4}{5} = - \frac{{33}}{{65}}\)