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 $-cos 30°$ .

$cos 30° = sqrt3/2$

$-cos 30° = -sqrt3/2$

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