已知函数f(x)=2sin(1/3x-30°）,x属于R,求f(5π/4)的值

(1)直接代入计算就是了,x/3-π/6=π/4所以f(x)=√2
(2)代入条件可得：sinα=5/13,sin(β+π/2)=cosβ=3/5,由于α,β∈[0,π/2],
cosα=12/13.sinβ=4/5,
cos(α+β)=cosαcosβ-sinαsinβ=16/65