Boa Noite !!!
[tex]\cfrac{11\pi }{2} \\\\\\\cfrac{11 . 180}{2} \\\\\\\cfrac{1980}{2} \\\\\\\cfrac{11\pi }{2} = 990^o, sabendo\ disso :\\\\\\\cfrac{990^o}{360^o} = 270^o, diminuindo\ ao\ primeiro\ quadrante :\\\\\\360^o - 270^o = 30^o,\ para\ o\ cosseno\ equivale :\\\\\\cos = \cfrac{\sqrt{3} }{2} \\\\\\[/tex]
Espero ter ajudado :-)
Att: LDC
