Cotangent of a Sum: Discovery

The applet below contains a rectangle with 2 similar triangles. The upper right segment has length 1.  There are 7 expressions shown off to the right.   Your job is to carefully drag each expression (label) next to the segment (in the figure) whose length is given by this expression.  Don't bother labeling the dashed hypotenuse of the right triangle in the middle.  Leave this dashed segment blank (without a label.)  

Study the following diagram carefully. Can you write an expression for ?