Find the exact value of the given expression using a sum or difference formula.

1) sin345°

2) cos (11π) / 12 ... ?

sin (a - b) = sinacosb - sinbcosa

1) sin345° = sin (360 - 15) = sin360cos15 - sin15cos360 = 0-sin15 * 1=

sin345° = - sin15,

2) cos (11π) / 12 = - cos pi / 12 = cos ( - 4pi/12 + 3pi/12) = cos ( - pi/3 + pi/4) =

cos (-pi/3) cos pi/4) + sin (-pi/3) sin (pi/4) = sqrt6 / 4 - sqrt2 / 4