What is the value of cos(-285) ?

The value of cos(-285) is (sqrt(2)(sqrt(3)-1))/4

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cos(-285)

cos (- 28 5^{\circ})

\frac{2 ( 3 - 1 )}{4}

Solution steps

cos (- 28 5^{\circ})

Use the following property:

cos (- x) = cos (x)

cos (- 28 5^{\circ}) = cos (28 5^{\circ})

= cos (28 5^{\circ})

Rewrite using trig identities:

cos (15 0^{\circ}) cos (13 5^{\circ}) - sin (15 0^{\circ}) sin (13 5^{\circ})

= cos (15 0^{\circ}) cos (13 5^{\circ}) - sin (15 0^{\circ}) sin (13 5^{\circ})

Use the following trivial identity:

cos (15 0^{\circ}) = - \frac{3}{2}

Use the following trivial identity:

cos (13 5^{\circ}) = - \frac{2}{2}

Use the following trivial identity:

sin (15 0^{\circ}) = \frac{1}{2}

Use the following trivial identity:

sin (13 5^{\circ}) = \frac{2}{2}

= (- \frac{3}{2}) (- \frac{2}{2}) - \frac{1}{2} \cdot \frac{2}{2}

Simplify

(- \frac{3}{2}) (- \frac{2}{2}) - \frac{1}{2} \cdot \frac{2}{2} : \frac{2 ( 3 - 1 )}{4}

= \frac{2 ( 3 - 1 )}{4}

sin (10 7^{\circ})

arctan (\frac{7}{20})

\frac{2 tan ( 6 0 ^{\circ} )}{1 - tan ^{2} ( 6 0 ^{\circ} )}

cot (\frac{1}{3})

sec (\frac{- 5 π}{4})

What is the value of cos(-285) ?
The value of cos(-285) is (sqrt(2)(sqrt(3)-1))/4