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February 1, 2015

February 1, 2015

Posted by **lina** on Tuesday, April 3, 2012 at 4:06pm.

cos(-13pi/12)

- pre-calculus -
**Steve**, Tuesday, April 3, 2012 at 5:31pmcos(-13pi/12)

= cos(-13pi/12 + 24pi/12)

= cos(11pi/12)

= -cos(pi - 11pi/12)

= -cos(pi/12)

now, cos(x/2) = √((1+cos(x))/2)

-cos(pi/12) = -√((1+cos(pi/6))/2)

= -√((1+√3/2)/2)

= -√(2 + √3/4)

= -1/2 √(2+√3)

or

-1/2√2 (1+√3)

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