Strange angle differentiation

Ok first time being here but I could use some help. The problem is this.

-There are 2 blood vessels seperated by angle X

-These two blood vessels have radii of R and r respectively

-The length "a" is the length of the primary blood vessel while the height "b" is the length from the end of the primary blood vessel to the branching vessel.

-The resistance to blood flow is as follows:

T= ((a-bcot(x))/R^4)-((bcsc(x))/r^4

-R,r,a, and b are all constants.

-Which angle "x" would provide the least resistance of blood?

I worked out the derivative to look like

T'=((bcsc(x)^2/R^4)+((bcsc(x)cot(x)/r^4)

But I have no idea how to get a minimum of resistance since the only angles to make this 0 or undefined are angles such as 0 90 or 180 which are not the answers. Have I done something wrong in my derivative? Is there a specific answer?