In both cases, you have to work on the angle between the sides you found out to be equal.
Diagram 1:
You can show that the angle PCB is the same as ACR. To prove this, you can think of angles:

and

So, angle
is common, and both
and
are right angles.
So, angles PCB and ACR are both 90° plus a common term (ACB), and thus they're equal.
Diagram 2:
We will work very similarly here: in this case, you have

and

Again, both
and
are right angles, so we have

and

So, both
and
are 90 minus a common term, so they are equal.