8.cosB=3/5>0，B为锐角，sinB>03/5<√2/2，π/4A=π-B-C，A<3π/4sinA=5/13<1/2，0又A<3π/4，因此00sinB=√(1-cos²B)=√[1-(3/5)²]=4/5cosA=√(1-sin²A)=√[1-(5/13)²]=12/13cosC=-cos(A+B)=sinAsinB-cosAcosB=(5/13)(4/5)-(12/13)(3/5)=-16/65cosC的值为-16/65
