Posted by **Jon** on Sunday, November 4, 2012 at 8:08pm.

how to find the exact value of: cos(9*pi/12)*cos(5*pi/12)+sin(9*pi/12)*sin(5*pi/12)

- Pre-calculas Math -
**Sanya**, Sunday, November 4, 2012 at 9:04pm
9pi/12 should be converted into degrees. The value of pi is 180 so all you do is 180 times 9 divided by 12 and then take the cosine of that value.

- Pre-calculas Math -
**Steve**, Monday, November 5, 2012 at 1:53am
cosx cosy + sinx siny = cos(x-y)

so, what we have here is

cos(9π/12 - 5π/12) = cos(4π/12) = cos π/3 = 1/2

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