Angle b/w Planes: Find theta in Radians

[Colts]

Homework Statement

Consider two planes with normal vectors n1=<0,-1,-2> and n2=<-2,1,-2> For these planes and angle theta between them

Homework Equations

n1.n2=||n1|| ||n2|| cos(theta)

The Attempt at a Solution

I have gotten my answer for theta to be .729727 radians. Not sure why this isn't right.

 

[vela]

You need to show your work. We can't tell where you're making mistakes if all you do is tell us your final, wrong answer.

 

[Colts]

n1.n2=<0,-1,-2>.<-2,1,-2> = -1-4=-5
||n1|| = sqrt(5)
||n2|| = sqrt(9)
pi-cos^-1(-5/sqrt(45)) = .729727

 

[vela]

(-2)x(-2) = +4

 

[Colts]

n1=<0,-1,2> not -2 sorry

 

[vela]

Try using simply the arccosine instead of pi minus the arccosine to get an answer.

 

[Colts]

2.41186 is the answer I got and that's not right either.

 

[vela]

Well, you're doing the calculation correctly, so there's something else that's wrong.

 

[Colts]

Yeah. It's only homework. I'm not a fan of math homework online. Inputs are tricky sometimes. Thanks for the help