We have to find the value of cos 210
Solution
Cos 210 can be found out using cos 180 degrees
cos 210 can be expressed as
cos 210= cos (180 + 30)
We know that cos (180 +x) = – cos x
Hence
cos 210= cos (180 + 30)
= -cos 30
We have learnt that the value of cos 30 = – √3/2
Answer
The value of cos 210= – √3/2