cosA=4/5,所以sinA=3/5 cosB=15/17,所以sinB=8/17 sinc=sin(180-A-B)=sin(A+B)=sinAcosB+sinBcosA=(3/5) * (15/17)+(4/5)*(8/17)=77/85 cosC=[1-(sinC)^2]的平方根=36/85