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All to do with properties of triangles rather than electrical engineering.
Look at my re-hashed version of your diagram.
There is always 180 degrees in a triangle.
Consider the trangle ADC, if angle CAD is Phi2 and angle ADC is a right angle, then angle ACD is (90 - Phi2).
Now consider the triangle ABC, if angle ACB is a right angle (90deg), we already know that angle ACD is (90 - Phi2), then angle BCD must be (90 - (0 - Phi2)) which is equal to Phi2.
JimB