How do you evaluate #cos(-210)#?

2 Answers
Aug 18, 2016

#cos(-210^@)=-sqrt3/2#.

Explanation:

We know that, #(1) : cos(-theta)=costheta, &, (2) : cos(180^@+theta)=-costheta#.

Hence, #cos(-210^@)=cos(210^@)=cos(180^@+30^@)=-cos30^@=-sqrt3/2#.

# - cos 30° = -sqrt3/2#

Explanation:

# -210°# means the line is rotating in an anticlockwise direction though 210 degrees. It's value is equal to # +150°#.

cos #150°# =# cos (180-30)°#.

It's value is the same as # -cos 30°#.

#cos 30° = sqrt3/2#

#-cos 30° = -sqrt3/2#