由题意可设cosa=3/5,cosb=-12/13则原式=cos(a+b),又因为a,b的范围为[0,π）所以sina=4/5,sinb=5/13所以cos(a+b)=cosacosb-sinasinb=-36/65-20/65=-56/65所以cos[arccos3/5+arccos(-12/13)]=-56/65...