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

2 Answers
Ratnaker Mehta
Aug 18, 2016

cos(-210^@)=-sqrt3/2cos(−210∘)=−√32.

Explanation:

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

Hence, cos(-210^@)=cos(210^@)=cos(180^@+30^@)=-cos30^@=-sqrt3/2cos(−210∘)=cos(210∘)=cos(180∘+30∘)=−cos30∘=−√32.

Mahendra S. · EZ as pi
Aug 18, 2016

- 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

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