Converting a Cartesian expression to polar coordinates

[hermano]

Homework Statement

Hi, I have the coordinates of an "expression" for a point in a cartesian coordinate system. I'm trying to write it in a polar coordiante system (in function of r and theta) but I don't know how to find the answer

a = y-component of the point
b = x-component of the point
theta = angle between the y-axis and a line connecting the point with the origin
r = distance from the origin till the point

Homework Equations

the expression which I would like to write in a polar coordinate system is:

a.sin(theta) + b.cos(theta)

The result should be:

r.sin(theta).cos(theta)

How to get to this result?

The Attempt at a Solution

 

[jncarter]

Start off drawing your unit circle. You should be able to figure it out from there. If you can't figure out the transform, trying looking up coordinate transforms on wikipedia. I know they have a good page on the topic.

If you have a problem understanding where the transforms come from or how to use them, try going over some examples in a textbook or drawing the vectors. This is something one really needs to understand to perform well in physics.