Evaluate exactly:

(a)  sin(345)=\displaystyle {\sin{{\left(-{345}^{\circ}\right)}}}=   

(b)  cos(15)=\displaystyle {\cos{{\left({15}^{\circ}\right)}}}=  

(c)  tan(255)=\displaystyle {\tan{{\left({255}^{\circ}\right)}}}=