Expert: Scott A Wilson - 10/22/2009

Question
I need help in writing proofs.  I don't have a clue as to how to write them out.  I can look at a diagram and see from the markings that 2 lines are parallel and that it is cut by a transversal.  I know about alternate angles and interior angles.  But how do I prove it?  Is there a specific order for the proof to be in?  and what is reflexive?  To me, it just repeats itself.  Help if you can

Answer
Lets call the angle in question A.

The angle write below it is B, and it is known that since A+B makes for the line,
that A+B=180°.

Now consider the angle opposite of A and call it angle C.

Since B and C form a line, it is known that B+C=180°.

Taking both equations, it is known that A = 180° - B and C = 180° - B.

Geometrically, it could be said that I have shown that B is the supplement of A and the supplement of C.  Since both C and A are supplements to B, they must be equal.