b=a+2,c=a+4
则（a+4)^2=a^2+(a+2)^2-2a(a+2)cos(2/3π)
a=3
所以c=7
(2)过点A作BC的垂线,垂足是D
则AD=√3sinθ,BD=√3cosθ,
则AC=2sinθ,CD=sinθ,BC=√3cosθ-sinθ,
△ABC的周长=√3+2sinθ+√3cosθ-sinθ
=√3+sinθ+√3cosθ
=√3+2[(1/2)sinθ+(√3/2)cosθ]
=√3+2[cos(1/3π）sinθ+sin(1/3π)cosθ]
=√3+2sin(1/3π+θ)
当θ=1/6π时,周长有最大值√3+2