1. The problem statement, all variables and given/known data Consider two planes with normal vectors n1=<0,-1,-2> and n2=<-2,1,-2> For these planes and angle theta between them 2. Relevant equations n1.n2=||n1|| ||n2|| cos(theta) 3. The attempt at a solution I have gotten my answer for theta to be .729727 radians. Not sure why this isn't right.