Question

1)cos 780;sin 13П/6-Вычислить. 2)Найти:sin a,если cos a=-12/13;П

Answer 1

$1) cos780^0=cos(720+60)=cos(2 pi +60)=cos60^0= frac{1}{2}$


$sin frac{13 pi }{6}=sin( frac{12 pi }{6}+ frac{ pi }{6})= sin( 2 pi + frac{ pi }{6})=sin frac{ pi }{6}= frac{1}{2}$

$pi textless alpha textless frac{3 pi }{2} \ \ sin alpha =-sqrt{1-cos^2 alpha }= - sqrt{1-( -frac{12}{13})^2 } =- sqrt{ frac{169-144}{169} } =- frac{5}{13}$