# Homework Help: A Cosine Law question involving angle of depression

1. Aug 4, 2011

### Gracegao

Smart people help! Trignometric question.

1. The problem statement, all variables and given/known data
A pedestrian bridge is build over a river. The angle of depression from one end of the bridge to a large rock beside the river is 37°. The distance from that end of the bridge (ptA) to the rock is 112m while the distance from the rock to the other end of the bridge(ptB) is 75m. Determine the length of bridge.

2. Relevant equations
a^2=b^2+c^2-2bcCosA

3. The attempt at a solution
I;m not sure about how to draw the diagram. The angle between the distance from the rock to A and the distance from the rock to B is 143° (180°-37°）

Last edited: Aug 4, 2011
2. Aug 4, 2011

### SammyS

Staff Emeritus
Does this triangle only have two angles?

--- or isn't it a triangle?

3. Aug 4, 2011

### uart

Re: Smart people help! Trignometric question.

That's not the usual definition of "angle of depression". So is that your working or is that actually part of the problem statement. If that really is where the 37 degree angle is measured then it's a simple cosine rule problem. If however the 37 degrees really corresponds to the angle of depression of the measurement from A to the rock then there is insufficient information to solve this problem.

Look up the definition of "angle of depression".