已知函数f(x)=2sin(1/3x-π/6)
问题描述：
已知函数f(x)=2sin(1/3x-π/6)
（1）求f(5π/4)
(2)设α,β∈[0,π/2],f(3α+π/2)=10/13,f(3β+π/2）=6/5.求cos(α+β)的值.
答
（1）π=180°所以f(5π/4) =2sin（5/4*1/3π-π/6）=2sin45°=根号2（2）f(3α+π/2)=2sin(α+π/6-π/6)=2sinα=10/13 sinα=5/13 f(3β+π/2）=2sinβ=6/5 sinβ=3/5因为sin^2(α)+cos^2(α)=1且α,β∈[0,π/2],...第二小题错了，f(3β+π）=6/5.求cos(α+β)的值.