## Triangle Proofs

#### Aligned To Common Core Standard:

**High School Congruence** - HSG-CO.C.10

What are Triangle Proofs? A triangle is a three-sided polygon which can be classified by its angles and sides. When classifying we are based on their sides, you should see if any sides are of the same lengths. If none of the sides are equal, then it is known as scalene. If any two lengths are the same then it is known as isosceles. If all three sides are of the same length, then it is known as the equilateral. When classifying the triangle based on angles, you look at the size of angles. If the triangle has a 90° , then it is known as a right. If all the angles of a triangle measure less than 90° then it is known as an acute. And if the measure of one of the angles is greater than 90° , then it is known as an obtuse. If you were asked to prove that sum of the interior angles of a triangle is 180 degrees, you would need to be familiar with this property of a triangle. Below we have discussed the proof for this property. Consider a triangle ABC: Using parallel postulate, there is one line parallel to side AC passing through B. ∠DBA is congruent to ∠A as they are alternate interior angles, and these angles are congruent when lines are parallel. Therefore, ∠DBA is equal to ∠A. Similarly, ∠EBC is congruent to ∠C as they are also alternate interior angles and so ∠EBC is equal to ∠C. ∠DBA + ∠ABC + ∠EBC = 180. These worksheets and lessons help students to learn how to start writing and creating proofs of triangles of their very own. This takes some practice and can be intimidating at first, but it gets easy with practice.

### Printable Worksheets And Lessons

- Missing 3-D Angles Step-by-step
Lesson - We get you into the rhythm of stating proofs and working
them in order.

- Guided Lesson - This
is a very practical skill is you ever take up and form of construction.

- Guided Lesson Explanation
- All the proofs are stated long hand to make sure you don't miss
a step.

- Practice Worksheet -
I couldn't believe that I got all problems on one page. I'm trying
to do that more often.

- Matching Worksheet
- I was able to get three questions out of one scenario for this
one.

- Indirect Euclidean Proofs Worksheet
Five Pack - Figure the reasoning behind the statements that
are presented to you.

- Indirect Euclidean Proofs Worksheet
Five Pack - You are some what working backwards with this one.

- Beginning Congruent Triangle
Proofs Worksheet Five Pack - These are novice level proofs.

- Practice with Beginning Congruent
Triangle Proofs Worksheet Five Pack - The heat gets turned up
a bit here.

- Proofs Involving Congruent
Triangle Worksheet Five Pack - I definitely gave a little too
much information here in the given areas. I would encourage you to take your time with the given.

- Proofs Involving Congruent Triangle
Worksheet Five Pack - These given areas are pretty light.

- Pre-Proof Warm-ups with Definitions Worksheet
Five Pack - What conclusion does each problem lead you to?

- Similarity of Triangles with
Similarity Proofs Worksheet Five Pack - I thought this was a
cool way to do it. Tell me which information is actually helpful. State which property is used to prove
similarity.

#### Homework Sheets

Bisectors appear for the first time in this section.

- Homework 1 - Use that that given so that you can prove that line PR bisects ∠QPS & ∠QRS. You can use the theorem ASA.
- Homework 2 - When two congruent angles are bisected, it leaves you with a lot about the system. Use it to your advantage.
- Homework 3 - It is interesting how angles from different sides of the track are congruent.

#### Practice Worksheets

Remind students that drawings are not drawn to scale. This is important to keep in mind.

- Practice 1 - Line VY is a mid segment of the ▲ UWX. If WX= 6, Prove VY=3. This throws them at first, just remind them it is not to scale.
- Practice 2 - Midpoints are found at the halfway mark. We learn the difference between midpoints and midsegments.
- Practice 3 - UW is a mid segment of ▲ VXY. If UW = 5, prove XY= 10

#### Math Skill Quizzes

These are some of the most open ended questions that you will see in geometry.

### Tips for Working with Triangle Proofs

You always what to make sure that you completely consider all the information that you are provided with prior to starting anything. This is often called the given, make sure to rewrite it. This will often be the first step of these types of problems. It may require you to rearrange and restate this material evidence slightly differently than it has been presented to you. In most cases, you will start by proving some form of congruency which can be inferred from the given. If congruency is not blatantly provided for you in the given, so what you can relate to the given to make it so. This can be done through a number of different theorems that relate to triangle geometry such as ASA, AAS, HL, SAS, or SSS. Once congruency is proven, you can infer the measures of related parts by just referring to basis of theorem itself. The foundation is based on the fact that corresponding parts of congruent triangles are themselves congruent. In general, you will find that following this approach will help you provide solid proofs four out five times.