Question 107747
the location of the vertex determines the vector ... vAC is the negative of vCA, etc.


vAB=(3,1,-3) ... vAC=(2,-2,0)
... vAB dot vAC=6-2+0=4
... cos(BAC)=4/(sqrt(19)*sqrt(8))


vBA=(-3,-1,3) ... vBC=(-1,-3,3)
... vBA dot vBC=3+3+9=15
... cos(ABC)=15/(sqrt(19)*sqrt(19))


vCA=(-2,2,0) ... vCB=(1,3,-3)
... vCA dot vCB=-2+6+0=4
... cos(ACB)=4/(sqrt(8)*sqrt(19))