等腰△ABC内A为顶角,已知sinB=8/17 求cosA sinB tanA

①等腰△ABC内A为顶角,sinB=8/17,得 0＜B=C＜π/4 A=π-2B＞π/2
∴ cosB=15/17 sinA=sin2B=2sinBcosB=240/289 cosA=-161/289
tanA=sinA/cosA=-240/161 cosA*sinB*tanA=sinB*sinA=1920/4913
②由cosA =15/17 cosB=9/41,得 sinA=8/17 sinB=40/41
∴ cosC=cos[π-(A+B)]=-cos(A+B)=-cosAcosB+sinAsinB=185/697