（1）∵cosα =sin（π/2＋α）
∴cos(4x-π/6)=sin（π/2+4x-π/6）=sin(4x+π/3)
f(x)=2sin(4x+π/3)
MAX=2 MIN=-2
递减区间π/2+2kπ≤x≤3π/2+2kπ
π/24+1/2*kπ≤x≤7π/24+1/2*kπ
递增区间-5π/24+1/2*kπ≤x≤π/24+1/2*kπ
（2）g(x)=2sin[4(x+a)+π/3]
象关于y轴对称,得到g(x)=2cos4x
则4a+π/3=π/2+2kπ
a最小真值=π/24