Question: What is the inverse of the function 

```
math.atan2

```

I use this in Lua where I can get the inverse of `math.atan` by `math.tan`.  

But I am lost here.

**EDIT**

OK, let me give you more details.

I needed to calculate angle between 2 points (x1,y1) and (x2,y2) and I did,

```
local dy = y1-y2 
local dx = x1-x2
local angle = atan2(dy,dx)* 180 / pi

```

Now if I have the angle, is it possible to get back dy and dx?

Answer: 
Apparently, something like this will help:

```
x = cos(theta)
y = sin(theta) 

```

Simple Google search threw this up, and the guy who asked the question said it solved it.
You'll probably get the wrong numbers if you use:

```
local dy = y1-y2 
local dx = x1-x2
local angle = atan2(dy,dx) * 180 / pi

```

If you are using the coordinate system where y gets bigger going down the screen and x gets bigger going to the right then you should use:

```
local dy = y1 - y2
local dx = x2 - x1
local angle = math.deg(math.atan2(dy, dx))
if (angle < 0) then
  angle = 360 + angle
end

```

The reason you want to use this is because atan2 in lua will give you a number between -180 and 180. It will be correct until you hit 180 then as it should go beyond 180 (i.e. 187) it will invert it to a negative number going down from -180 to 0 as you get closer to 360. To correct for this we check to see if the angle is less than 0 and if it is we add 360 to give us the correct angle.