记为等价符号
cos2A/(a*a)-cos2B/(b*b)=1/(a*a)-1/(b*b)
(2*cosA*cosA-1)/(a*a)-(2*cosB*cosB-1)/(b*b)=1/(a*a)-1/(b*b)
(cosA*cosA-1)/(a*a)=(cosB*cosB-1)/(b*b)
sinA*sinA/(a*a)=sinB*sinB/(b*b)
sinA/a=sinB/b
正弦定理