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January 28, 2015

January 28, 2015

Posted by **Sabrina** on Wednesday, November 17, 2010 at 12:56am.

cos (13pi/12)

PLeaseeeeeeeeeee help!!! thanx a million

- Trig math -
**Reiny**, Wednesday, November 17, 2010 at 10:12amSometimes it is easier for students to see these relationships in degrees.

13π/12 radians = 195°

and 195 = 45+150

Knowing the ratio of sides of the 30-60-90 and the 45-45-90 triangles we can find that

sin 150°=1/2

cos 150° = -√3/2

sin 45° = cos 45° = 1/√2 = √2/2

then back to

cos(13π/12)

= cos(195)

= cos(150+45)

= cos150 cos45 - sin150 sin45

= (-√3/2)(√2/2) - (1/2)(√2/2

= (-√6 - √2)/4

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