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: 
Given only the angle you can only derive a unit vector pointing to `(dx, dy)`. To get the original `(dx, dy)` you also need to know the length of the vector `(dx, dy)`, which I'll call `len`. You also have to convert the angle you derived from degrees back to radians and then use the trig equations mentioned elsewhere in this post. That is you have:

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

```

Given `angle` (in degrees) and the vector length, `len`, you can derive `dx` and `dy` by:

```
local theta = angle * pi / 180
local dx = len * cos(theta)
local dy = len * sin(theta)

```
As atan2 works as tan-1, so the inverse could be tan, taking into consideration conversion between radian and degree