Trig math
posted by Sabrina .
how to split up using sum and difference identities
cos (13pi/12)
PLeaseeeeeeeeeee help!!! thanx a million

Sometimes 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 306090 and the 454590 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