已知平面向量a=(sinα-1,1) 向量b=(1,1-cos) 向量a·b=1/5 a∈(0,π/2)

1、a·b=sinα-1+1-cosα=sinα-cosα=1/5
联立sin^2α+cos^2α=1,α∈(0,π/2)
∴sinα=4/5,cosα=3/5,sin2α=24/25
2、f(x)=5cos(2x-α)+cos2x
=5(cos2xcosα+sin2xsinα)+cos2x
=4(sin2x+cos2x)
=4√2sin(2x+π/4)
∴f(x)最小正周期=2π/2=π,对称中心横坐标为：2x+π/4=kπ,即x=kπ/2-π/8,坐标为(kπ/2-π/8,0)
对称轴为：2x+π/4=(2k+1)π/2,即x=kπ/2+π/8
3、x∈[-11π/24,-5π/24],2x+π/4∈[-3π/4,-π/6],
∴f(x)=4√2sin(2x+π/4)∈[-4√2,-2√2],根据4√2sinx图像得出

是-1+sina还是sin（a-1）?