（1）f(x)=2cos(x+π/6)+2sinx=√3cosx-sinx+2sinx=√3cosx+sinx
∵sinx=4/5,x∈[π/2,π]
∴cosx=-3/5
∴f(x)=√3cosx+sinx=-3√3/5+4/5
（2）f(x)=√3cosx+sinx=2sin(x+π/3)
∵x∈[π/2,π]
∴x+π/3∈[5π/6,4π/3]
∴f(x)∈[-√3,1]