用和差化积公式.
y=sin(π/3-2a)+cos(π/3+2a)
=sin(π/3-2a)+sin(π/3+2a+π/2)
=2sin(π/3+π/4)cos(2a-π/4)
所以最大值是2sin(7π/12)=(√6-√2)/2,周期是π