Question

cos 11п/3 ctg(-21п/4)=

Answer 1

$cos \frac{11 \pi }{3} *ctg \frac{-21}{4}= \frac{1}{2}*(-1)=- \frac{1}{2}$

Answer 2

$cos \frac{11 \pi }{3} *ctg (-\frac{21 \pi }{4})=-cos \frac{11 \pi }{3} *ctg\frac{21 \pi }{4}= -cos(4 \pi - \frac{ \pi }{3} )*ctg (5 \pi + \frac{ \pi }{4} )=$$=-cos \frac{ \pi }{3}*ctg \frac{ \pi }{4} =- \frac{1}{2} *1=-0.5$