Basic Geometry : How to find if right triangles are congruent

Study concepts, example questions & explanations for Basic Geometry

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Example Questions

Example Question #1481 : Plane Geometry

Are these right triangles congruent?

Congruent right triangles

Possible Answers:

Cannot be determined - we need at least one pair of angles, or all three sides 

No - at least one pair of corresponding sides is not congruent 

No - the angles are different

Yes - all three pairs of sides must be congruent by Pythagorean Theorem

Yes - by the angle-angle-side theorem

Correct answer:

Yes - all three pairs of sides must be congruent by Pythagorean Theorem

Explanation:

Right now we can't directly compare these triangles because we do not know all three side lengths. However, we can use Pythagorean Theorem to determine both missing sides. The left triangle is missing the hypotenuse: 

The right triangle is missing one of the legs:

subtract 2,304 from both sides

This means that the two triangles both have side lengths 48, 55, 73, so they must be congruent.

 

Example Question #11 : How To Find If Right Triangles Are Congruent

The hypotenuse and acute angle are given for several triangles. Which if any are congruent? Triangle A- Hypotenuse=15; acute angle=56 degrees. Triangle B- Hypotenuse=18; acute angle=56 degrees. Triangle C-Hypotenuse=18; acute angle= 45 degrees.

Possible Answers:

A & B

B & C

All three.

A & C

None of these

Correct answer:

None of these

Explanation:

The correct answer is none of these. There are several pairs of angles and sides or sides and angles that must be the same in order for two triangles to be congruent.

In our case, we need the acute angle and the hypotenuse to both be equal. No two triangles above have this relationship and therefore no two are congruent. 

Example Question #1483 : Basic Geometry

Given:  and .

 and  are both right angles.

True or false: From the above information, it follows that .

Possible Answers:

True

False

Correct answer:

True

Explanation:

If we seek to prove that , then , and  correspond to , and , respectively.

By the Hypotenuse-Leg Theorem (HL), if the hypotenuse and one leg of a triangle are congruent to those of another, the triangles are congruent. 

 and  are both right angles, so  and  are both right triangles.  and  are congruent corresponding sides, and moreover, since, each includes the right-angle vertex as an endpoint, they are congruent corresponding legs.  and  are opposite the right angles, making them congruent corresponding hypotenuses.

The conditions of HL are satisfied, so .

Example Question #302 : Right Triangles

Given:  and .

 and  are both right angles. 

True or false: From the given information, it follows that .

Possible Answers:

False

True

Correct answer:

False

Explanation:

The congruence of  and  cannot be proved from the given information alone. Examine the two triangles below:

Triangles 2

, and  and  are both right angles, so the conditions of the problem are met; however, since the sides are not congruent between triangles - for example,  - the triangles are not congruent either. 

 

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