Calculate Angle between Two Intersecting Line Segments Need some help/direction, haven't had trig in several decades.
On a 2 dimensional grid, I have two line segments.
The first line Segment always starts at the origin $(0,0)$, and extends to $(1,0)$ along the $X$-axis.
The second line Segment intersects the first at the origin, and can extend to potentially anywhere within $(-1,-1)$ to $(1,1)$.
I need to always calculate the angle to the right of the first segment... 
If this is already addressed in another post, please comment me the link.
UPDATE
I will have a single input of $(x,y)$ for the end of the 2nd segment... 
so segment $A$ would be $(0,0)$ →$ (1,0)$ and segment $B$ would be $(0,0)$ → $(x,y)$ where $(x,y)$ can be anywhere inside $(-1,-1)$ and $(1,1)$ assuming that the scale is $0.1$.
Let me know If I can provide any additional information that will help.
UPDATE
OK... assuming that the first segment is running along the $Y$-axis... $A(0,0)$ and $B(0,1)$
And the second segment is running from $A(0,0)$ to $C(.4,.4)$ with a scale of .2....
$$\theta= \tan^{-1}{\dfrac{.4}{.4}}= 45$$
If I change C to $C(.4,-.4)$ I get.
$$\theta= \tan^{-1}{\dfrac{.4}{-.4}}= -45$$
Do I have to manually compensate for the quadrant because this seems to calculate based specifically on the axis... I would expect the 2nd one to come up as 135 degrees from the positive Y Axis...
What am I missing?
Just for posterity...
If I had $C(-0.4,-0.1)$ I would expect the result for the angle from the positive Y axis to this line segment to be roughly 255 degrees... 
$$\theta= \tan^{-1}{\dfrac{.4}{-.1}}= 75.9637$$
Plus 180 from starting at the positive Y axis....

Not sure EXACTLY what you are asking, but I will answer this to the best of my ability. If you could include a visual that would greatly help me.
When we have two intersecting line segments like this

finding any single value (a, c, b, d) will reveal all other values. For example, if we have the value of a, then c = a, and we have (in degrees) b = d = 180-a.
Therefore, I can equip with the tools to find the angle between two vectors, as you have given. For example, we can treat the first vector as you have said as $[1, 0]$ and the second vector as $[1, 1]$. We take the dot product between them, which just means that we multiply the corresponding values and sum them up, or $<a, b> = \sum\limits_{i = 1}^{n}{a_ib_i}$ where $n$ is the number of elements in the vector. We use the geometric fact that $<a, b> = |a||b|cos(\theta_{a, b})$ where $|a|$ means its norm or magnitude (I only deal with standard inner product and norm here). This gives us that the dot product, which is $1$, is equal to $\sqrt{2}cos(\theta)$, which means $cos(\theta) = \frac{1}{\sqrt{2}}$
This in turn gives us that the angle between these two is 45 degrees, and we can figure out the adjacent angle as 135, and the vertical angles all share the same degrees.
