What is the value of cos(1290) ?

The value of cos(1290) is -(sqrt(3))/2

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cos(1290)

cos (129 0^{\circ})

- \frac{3}{2}

Solution steps

cos (129 0^{\circ})

cos (129 0^{\circ}) = cos (21 0^{\circ})

= cos (21 0^{\circ})

Rewrite using trig identities:

cos (18 0^{\circ}) cos (3 0^{\circ}) - sin (18 0^{\circ}) sin (3 0^{\circ})

= cos (18 0^{\circ}) cos (3 0^{\circ}) - sin (18 0^{\circ}) sin (3 0^{\circ})

Use the following trivial identity:

cos (18 0^{\circ}) = (- 1)

Use the following trivial identity:

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

Use the following trivial identity:

sin (18 0^{\circ}) = 0

Use the following trivial identity:

sin (3 0^{\circ}) = \frac{1}{2}

= (- 1) \frac{3}{2} - 0 \cdot \frac{1}{2}

Simplify

= - \frac{3}{2}

tan (arcsin (\frac{4}{5}) - arccos (1))

tan (arcsin (\frac{2}{3}) - arccos (\frac{1}{3}))

cos (30 0^{\circ}) cos (1 5^{\circ}) - sin (30 0^{\circ}) sin (1 5^{\circ})

2 cos (2 π)

\frac{0.5}{sin ( 0.5 )}