SinC+Sin(B-A)=2sin2A
sin（B+A）+sin(B-A)=2sin2A
sinBcosA+cosBsinA+sinBcosA-cosBsinA=2*2sinAcosA
化简得 ：sinBcosA = 2sinAcosA
若 cosA=0 , 即 A=90°则三角形ABC为Rt三角形
则 a=4√3/3 b =2√3/3
S = 1/2 * ab cosC = 2√3/3
若 cosA 不为 0 则 sinB =2sinA 则 b=2a
因为余弦定理 c2=a2+b2-2abcosC则 a= 2√3/3b = 4√3/3
S= 1/2 * ab cosC = 2√3/3