一道数学题，急需，在线等

因为sinb = 3/5且π/2<者卖汪b<π,所以cosb = -4/5
因此tanb = sinb/cosb = -3/4
因配芹为sin(a+b) = sinacosb + sinbcosa = cosa
所以sinacosb = cosa(1-sinb) = cosa*(1-3/5) = (2/5)cosa
所以tana = 2/(5cosb) = -1/2
所以tan(a+b) = (tana+tanb)/(1-tanatanb)
= (-1/2 -3/4) /首仔 (1-(-1/2)(-3/4))
= -(5/4) / (1-3/8)
= -2

错了哈、。。

25/12

由已知sinb=3/5，π/锋凯誉2

易知cosb=-4/5

sin(a+b)=cosa
sina cosb+cosa sinb =cosa
-4/5 sina +3/5 cosa =cosa
-4/5 sina =-2/5 cosa
2sina=cosa

tan(a+b)=sin(a+b) / cos(a+b) =cos a /裂旦磨肆斗cos(a+b) =cosa /迟颂[cosa cosb -sina sinb]
=2sina /[2sina (-4/5) -sina (3/5)]
=2/[-11/5]
=-10/11

因为sinb=3/5,π/2因为sin(a+b)=cosa
所以sinacosb+sinbcosa=cosa
即配咐-4/5sina+3/5cosa=cosa
-4/5sina=2/5cosa
所以,tana=-1/2
tan(a+b)=(tana+tanb)/(1-tana*tanb)=-2