Question

COS(-150)=COS 150
-150 is in 3rd quadrant (clockwise movement for negative angles) where cos in negative then How in above equation cos has turned -150 into positive?

Answer 1

cos(-150°) = cos(360° - 150°) = cos(210°)

= cos(180° + 30)°

= -cos30° ... (i)

... (cosine in third quadrant is negative)

Now,

cos(150°) = cos(180° - 30°) = -cos30° ... (ii)

... (cosine in second quadrant is negative)

hence, from (i) and (ii)

cos(-150°) = cos(150°)