cos^2B=cos^2 (150-a)=(cos150cosa+sin150sina)^2
=(-cos30cosa+sin30sina)^2
=3cos^2 a /4 +sin^2 a/4 -根号3 sinacosa/2 =sin^2 a
3cos^2 a/4 -根号3 sinacosa /2 =3sin^2 a/4
3cos^2 a -2根号3 sinacosa =3sin^2 a
3(cos^2 a -sin^2 a) -根号3 sin2a=0
3cos2a -根号3sin2a=0
tan2a =根号3
所以2A=60A=30 B=120