Writing Triangle Proofs

Learning Target:Students can write 2 column proofs showing that triangles are congruent.

2 Column Proofs:

A deductive argument that contains statements and reasons organized in two columns.

The Basics… The point of writing proofs is to PROVE

that two triangles are congruent. There isn’t a set number of proofs that you

have to have to prove that two triangles are congruent.

The more proofs, the better. There isn’t a set order, just make sure that

the proofs make sense – sometimes one thing can’t happen before another.

One of the 5 Congruence Postulates must be proven

Step 1: Set up a two-column proof format. 

› The left-hand column should be labeled "Statements" and the right-hand column labeled "Reasons."

STATEMENT REASON

List your givens as the first step in your statements column. Under the reasons column, you should write "given."

Step 2:

Translate the givens into useful information. › For each given, you'll want to think what you could do with that knowledge. At this point in the proof, most of your reasons will be definitions.

Step 3:

› If one of the givens is "Point C is the midpoint of AB," your statement would read "AC = CB" and your reason would be "Definition of midpoint," or the full theorem: If a segment has a midpoint, then it divides the segment into two congruent parts.

Example #1:

› If segment XY bisects segment NM, and P is the point where XY intersects NM, your statement would read "NP = PM" and your reason would be "Definition of bisector," or the full theorem: If a segment is bisected, then it is divided into two congruent parts.

Example #2:

› If the givens say "KL is perpendicular to HJ," your statement would be "Angle KLH is a right angle." Your reason would then be: If two perpendicular lines intersect, then they form right angles. After this, you would reference the step that says "KL is perpendicular to HJ," and write the number in parentheses next to the reason. The letters will depend on the diagram, which is one reason why the diagram is important.

Example #3:

Look for any isosceles triangles in the diagram based on your givens.› Isosceles triangles are triangles that

have two legs of equal length. Write the two legs are congruent as a statement citing "Definition of isosceles" as the reason.

Step 4:

Look for parallel lines. › If a third line runs through both of them, it will

form alternate interior angles and corresponding angles. These are quite common in triangle proofs. The overall shape formed by the angles looks like a the letter "z.“ Write in your statements column that those angles are congruent to each other with the reason being "Alternate interior angles are congruent" or "Corresponding angles are congruent."

› Do not use reasons such as "Definition of parallel" or "Definition of alternate interior angles." These are not valid proof reasons.

Step 5:

Look in the diagram for vertical angles. › Vertical angles will form an X and are the angles

across from each other, touching at the center but not along their sides. These vertical angles are congruent. An example would be to use the statement "Angle P is congruent to angle R" with the reason "Vertical angles are congruent."

Step 6:

Look in the diagram for any lines or angles shared by triangles. › This is where the reflexive property comes into

play. Your statement should read "XY = XY" or "Angle A = angle A" and the reason would be "Reflexive property of congruence."

Step 7:

Transfer every congruency statement you've found so far, including the givens, into the diagram. › Congruent sides get marked with hash marks;

congruent angles are marked with arcs.

Step 8:

Look at one of the triangles in the picture. › Note all the markings you just made and the ones that were

given to you. How many angles are congruent? How many sides? Match this information with one of the triangle congruency theorems. Your statement should read "Triangle ABC is congruent to triangle XYZ" and the reason would be the appropriate choice between AAS, ASA, SAS, SSS, HL, etc.

› SSS stands for side, side, side. All three sides must be congruent.› Two sides and an angle must be congruent for it to be SAS (side,

angle, side).› See if your triangles have congruent right angles, and two

congruent sides, one being the hypotenuse (which is the side directly across from the right angle). If they do, then state them as being congruent with the reason HL (Hypotenuse Leg Theorem).

› It can be ASA or AAS if two angles are congruent, but only one side.

Step 9:

Take another look at the "Prove" line of your problem. › If it's the same as your last statement, then you

are finished.

Step 10: