Crossing a River: Find Speed Needed to Swim from A to B

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Homework Statement

A swimmer wants to cross a river, from point A to point B, as shown in the figure. The distance d_1 (from A to C) is 200m , the distance d_2 (from C to B) is 150m, and the speed v_r of the current in the river is 5 km/h. Suppose that the swimmer's velocity relative to the water makes an angle of $$\theta = 45 degrees$$ with the line from A to C, as indicated in the figure.

To swim directly from A to B, what speed u_s, relative to the water, should the swimmer have?

Homework Equations

I would think that you would use nothing more than some trig, Pythagorean theorem and stuff about adding and subtracting vectors

The Attempt at a Solution

I have no idea how to attempt this solution. I don't know where to even start. I know the answer is 4.04km/h but I have no idea how to get to that answer. Where should I start?

 

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Well don't all jump up at once now!

 

[Doc Al]

What direction must the swimmer's velocity have with respect to the shore? Hint: The velocity of the swimmer with respect to the shore = velocity of the swimmer with respect to the water + velocity of water with respect to the shore.