GMAT Math : Triangles

Study concepts, example questions & explanations for GMAT Math

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Example Questions

Example Question #2 : Dsq: Calculating Whether Right Triangles Are Similar

You are given two right triangles:  with right angle , and  with right angle 

True or false: 

Statement 1: 

Statement 2: 

Possible Answers:

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

Correct answer:

EITHER statement ALONE is sufficient to answer the question.

Explanation:

Assume Statement 1 alone is true. Then, since , both being right angles, and  from Statement 1,  follows by way of the Angle-Angle Similarity Postulate. A similar argument shows Statement 2 also provides sufficient information.

Example Question #3 : Dsq: Calculating Whether Right Triangles Are Similar

You are given two right triangles:  with right angle , and  with right angle 

True or false: 

Statement 1: The ratio of the perimeter of  to that of  is  to .

Statement 2: .

Possible Answers:

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

EITHER statement ALONE is sufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Correct answer:

BOTH statements TOGETHER are insufficient to answer the question. 

Explanation:

Assume both statements are true.

While in two similar triangles, the ratio of the perimeters, given in Statement 1, is indeed equal to that of the ratios of the lengths of the hypotenuses, given in Statement 2, this is not a sufficient condition for similarity. For example:

Case 1: 

Case 2:

In each case, the conditions of the main problem and both statements are met, since:

Both triangles are right - each Pythagorean triple is a multiple of Pythagorean triple 3-4-5;

The ratio of the perimeters is ; and,

.

But in Case 1, 

, since , and the similarity follows by way of the Side-Side-Side Similarity Principle. 

In Case 2, 

, since . This violates the conditions of similarity (note that in both cases, , but this is a different statement).

The two statements together are inconclusive.

Example Question #8 : Dsq: Calculating Whether Right Triangles Are Similar

Given:  and , where  and  are right angles.

True or false: 

Statement 1: 

Statement 2:  is an isosceles triangle.

Possible Answers:

BOTH statements TOGETHER are insufficient to answer the question. 

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

Correct answer:

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Explanation:

Each of Statement 1 and Statement 2 gives information about only one of the triangles, so neither statement alone is sufficient.

Assume both statements are true. From Statement 1,  and  is right and measures .

From Statement 2 alone,  is isosceles; the acute angles of an isosceles right triangle must both measure , so, in particular,  . Also, it is given that  is right.

 and  (both of the latter being right angles), and by the Angle-Angle Postulate, .

Example Question #511 : Data Sufficiency Questions

Given: Rectangles  and  with diagonals  and , respectively. 

True or false: 

Statement 1: 

Statement 2: 

Possible Answers:

EITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Correct answer:

EITHER statement ALONE is sufficient to answer the question.

Explanation:

Refer to the figure below, which gives the two rectangles with their diagonals as described.

Triangles and rectangles

Assume Statement 1 alone. The diagonals of a rectangle bisect the rectangle into congruent triangles, so  and . Congruent triangles are also similar, so it follows that  and . Since, by Statement 1,  - or, stated differently,  - by transitivity of similarity, 

, and

.

 

Assume Statement 2 alone. The quadrilaterals are rectangles, so , both being right angles. From Statement 2, , setting up the conditions of the Angle-Angle Postulate; therefore, .

Example Question #10 : Dsq: Calculating Whether Right Triangles Are Similar

Given: Rectangles  and  with diagonals  and , respectively. 

True or false: 

Statement 1: The perimeter of Rectangle  is three times that of Rectangle .

Statement 2: The area of Rectangle  is nine times that of Rectangle .

Possible Answers:

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

Correct answer:

BOTH statements TOGETHER are insufficient to answer the question. 

Explanation:

Assume both statements are true. We show the two statements together are insufficent to prove or disprove that 

Suppose 

.

Then Rectangle  has perimeter 

and area 

Now set up two cases with different dimensions for Rectangle .

Case 1:

The perimeter of Rectangle  is 

,

one-third that of Rectangle .

The area of Rectangle  is

one-ninth that of Rectangle .

,

so by the Side-Angle-Side Similarity Theorem, .

 

Case 2:

The perimeter of Rectangle  is 

,

one-third that of Rectangle .

The area of Rectangle  is

one-ninth that of Rectangle .

Sides are not in proportion, making the statement  false.

 

Example Question #2624 : Gmat Quantitative Reasoning

Given:  and , where  and  are right angles.

True or false: 

Statement 1: Both triangles are scalene.

Statement 2: The triangles have the same perimeter.

Possible Answers:

EITHER statement ALONE is sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

Correct answer:

BOTH statements TOGETHER are insufficient to answer the question. 

Explanation:

Assume both statements are true. Consider these two scenarios:

Let 

and 

Since  and , then by the Hypotenuse-Leg Theorem, , and it follows that 

 

Now, let 

and .

The sides are not in proportion, as , so .

In both scenarios, since , the triangles are right by the converse of the Pythagorean Theorem, satisfying the main premise; the triangles are scalene, having sides of three different lengths, satisfying Statement 1; and the triangles both have perimeter , satisfying Statement 2. The two statements together are insufficient.

Example Question #2625 : Gmat Quantitative Reasoning

Given:  and , where  and  are right angles.

True or false: 

Statement 1: Both triangles are isosceles.

Statement 2: The triangles have the same perimeter.

Possible Answers:

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

EITHER statement ALONE is sufficient to answer the question.

Correct answer:

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Explanation:

Assume Statement 1 alone. By the 45-45-90 Theorem, an isosceles right triangle has two acute angles that each measure ; specifically,  and . By the Angle-Angle Similarity Postulate, .

Assume Statement 2 alone. The equality of the perimeters of two right triangles does not imply their similarity or nonsimilarity. For example:

Let 

and 

Since  and , it follows by the Hypotenuse-Leg Theorem that , and, since congruent triangles are similar, it further follows that 

Now, let 

and .

The sides are not in proportion, as , so .

In both scenarios, since , by the converse of the Pythagorean Theorem, the triangles are right, and the triangles both have perimeter , thereby satisfying the main premise and Statement 2. Statement 2 alone is therefore insufficient.

Example Question #153 : Triangles

Given: Rectangles  and  with diagonals  and , respectively. 

True or false: 

Statement 1: Rectangles  and  are both rhombuses.

Statement 2: Rectangles  and  are both squares.

Possible Answers:

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

EITHER statement ALONE is sufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Correct answer:

EITHER statement ALONE is sufficient to answer the question.

Explanation:

Both rectangles, by definition, have four right angles.

A rhombus is a quadrilateral with four sides of equal length. A square is a quadrilateral with four right angles and four sides of equal length. Therefore, the two statements are actually equivalent in this context, so either they together provide insufficient information, or either alone does. We show that the latter is the case.

If both statements are true, and the rectangles are rhombuses and squares, then

 and .

Consequently,  

.

,

so by the Side-Angle-Side Similarity Theorem, .

Example Question #11 : Dsq: Calculating Whether Right Triangles Are Similar

Triangles and circles

Note: Figure NOT drawn to scale.

Refer to the above figure, which shows two right triangles circumscribed by circles.

True or false: 

Statement 1: The area of the circle that circumscribes  is four times the that of the circle that circumscribes .

Statement 2: 

Possible Answers:

EITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Correct answer:

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Explanation:

Assume Statement 1 alone. The area of the circle that circumscribes  is four times the that of the circle that circumscribes , so the radius of the former is the square root of this, or two, times the radius of the latter; therefore, the ratio of the diameters of the circles is also two. Since in a right triangle inscribed inside a circle, the hypotenuse must be a diameter of that circle, 

 or 

However, no other information is given or can be determined about any other sides or angles, so the triangles cannot been proved or disproved to be similar.

Assume Statement 2 alone. , or , but again, no other information is given or can be determined about any other sides or angles, so the triangles have not been proved or disproved to be similar.

Now assume both statements. , setting up the conditions for the Hypotenuse-Leg Similarity Theorem; it follows that .

Example Question #51 : Right Triangles

Triangle 2

Note: Figure NOT drawn to scale.

Refer to the above figure. True or false: .

Statement 1: 

Statement 2: 

Possible Answers:

EITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Correct answer:

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Explanation:

Assume Statement 1 alone. If , then corresponding angles are congruent - specifically, . Statement 1 directly contradicts this, so .

Statement 2 alone does not prove or disprove similarity, since information is only given about one set of corresponding sides. No information is given about corresponding angles, or any other set of corresponding sides. 

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