f(x) = cos(x+π/6) x ∈ [-π/2, π/2] = [-90°, 90°]
f(x) = cos(x+π/6) x+π/6 ∈ [-π/3, 2π/3] = [-60°, 120°]
********* 几个典型值 ************
cos(-60°) = 1/2 最左端
cos(0°) = 1 中间典型值 ----也是最大值
cos(90°) = 0 中间典型值
cos(120°) = -1/2 最右端 ----也是最小值
最小值是cos(π/2+π/6) = -sin(π/6) = -0.5
最大值是cos(-π/6+π/6) = cos(0) = 1
值域是[-0.5, 1]
『-1/2,1』