# Simple 3d trig problem

1. Jun 12, 2010

### cosmo123

1. The problem statement, all variables and given/known data
[PLAIN]http://img708.imageshack.us/img708/1344/trig.jpg [Broken]
The angle c is separated from a 2d plane by angle b (perpendicular to the 2d plane). Angle a is angle c projected onto the 2d plane. What is a in terms of b and c?

2. Relevant equations

3. The attempt at a solution
I put the whole thing in a cuboid, used the lengths of the edges, and came to tan(a)=tan(b)tan(c). Can i just double check this with someone, because i dont trust my maths.:grumpy:

Last edited by a moderator: May 4, 2017
2. Jun 12, 2010

### jack action

It doesn't work. When b = 0, no matter what c is, your equation gives a = 0, which doesn't make sense as a = c when b = 0.

This seems more appropriate to me: tan a = tan c / cos b

3. Jun 13, 2010

### cosmo123

Yeah, it doesnt work, i probably should have checked some figures. I re-did it and i think i missed a 1. I got tan a = sqrt((tan b)^2 + 1) * tan c

=> tan a = sec b * tan c

so yes, youre completely right, thanks.