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

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Answer 1 · 2016-08-18T12:10:20

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

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

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

Answer 2 · 2016-08-18T12:22:47

# - cos 30° = -sqrt3/2#

# -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#