sin(2a+b)/sina      -2cos(a+b)

sin(2a+b)/sina      -2cos(a+b)
sin(2a+b)/sina      -2cos(a+b)

sin(2a+b)/sina      -2cos(a+b)
原式=（sinacos(a+b)+cosasin(a+b))/sina-2cos(a+b)
=cosasina(a+b)/sina-cos(a+b)
=(cosasina(a+b)-sinacos(a+b))/sina
=sin(a+b-a)/sina
=sinb/sina

原式=(sin2a*cosb+cos2a*sinb)/sina -2(cosa*cosb - sina* sinb)
={2sina*cosa*cosb+(1-2sina的平方)sinb}/sina -2cosa*cosb+2sina*sinb
={(1-2sina的平方)/sina -2sina}sinb
=sinb/sina

好难啊