# Homework Help: Why is this certain angle 20 degrees?

1. Jun 13, 2010

### elementG

1. The problem statement, all variables and given/known data
Problem #199
http://img88.imageshack.us/img88/8008/scan0001vd.jpg [Broken]
Solution
http://img21.imageshack.us/img21/2131/199hg.jpg [Broken]

2. Relevant equations
Why is the angle from VB/A 20 degrees from the solution diagram? It would seem that I had to know that direction of VB/A had the same angle as VA in terms of the geometry (if two parallel lines are cut by a transversal, its alternating interior angles are equal). I just don't see how you can assume that.

3. The attempt at a solution
Since drawing a triangle is the first part, I don't have any "attempt" at it yet.

Last edited by a moderator: May 4, 2017
2. Jun 13, 2010

### DaveC426913

I'm not sure what you're asking.

You are given that the bearing is 20 degrees. i.e. A takes a bearing of B and sees it is 20 degrees East of North, thus theta is 20 degrees.

3. Jun 13, 2010

### elementG

I saw that 20 degrees was given, I just don't see how the angle is 20 degrees on the solution diagram. Ship A observes ship B at 20 degrees, but how is VB/A also 20 degrees down from horizontal? Sorry for the confusion!

4. Jun 13, 2010

### DaveC426913

It's been while, sorry, VB/A represents what part of the diagram?

I'd assumed we only care about angle theta, which is 20.

5. Jun 13, 2010

### elementG

VB/A comes off the head of VA. I just don't see how its 20 degrees when VB/A and VA are connected as seen on the solutions diagram.

6. Jun 13, 2010

### DaveC426913

Sorry, I hadn't looked at the second diagram.
Well, the solution triangle is just a rearrangement of the starting configuration. The two vectors start off at 20 degrees, why would that change?

7. Jun 13, 2010

### elementG

Oh, I guess I made the wrong assumption. I was assuming the angle that VB/A made was not necessarily 20 degrees. I guess I'm confused (a little bit) still is because I can't see it geometrically. Like say for instance, I'm still on the assumption that the angle is not 20 degrees for VB/A and I label as an unknown, how would I geometrically prove that the angle is 20 degrees?

8. Jun 13, 2010

### DaveC426913

You would not be able to solve the problem. You're given the angle because you need it.