GMAT Math : Geometry

Study concepts, example questions & explanations for GMAT Math

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Example Questions

Example Question #10 : Dsq: Calculating The Area Of An Equilateral Triangle

Which, if either, of equilateral triangles  and , has the greater area?

Statement 1: 

Statement 2: 

Possible Answers:

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.

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 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:

If we let  and  be the sidelengths of  and , respectively; the statements can be rewritten as:

Statement 1: 

Statement 2: 

Since the area of an equilateral triangle is dependent solely upon the length of each side, comparison of the lengths of the sides is all that is necessary to determine which triangle, has the greater area. The question can therefore be reduced to asking which of  and , if either, is greater.

Statement 1 alone is not sufficient to yield an answer, as we see by examining these two scenarios:

Case 1: 

Case 2: 

Both cases satisfy Statement 1, but in the first case, , meaning that  has greater sidelength and area than , and in the second case, , meaning the reverse. By a similar argument, Statement 2 is insufficient.

Now assume both statements to be true. The two equations together comprise a system of equations:

Multiply the second equation by  and add to the first:

            

                 

Now substitute back:

The sidelengths are the same, and, consequently, so are the areas of the triangles.

Example Question #21 : Equilateral Triangles

Given two equilateral triangles  and , which has the greater area?

Statement 1: 

Statement 2: 

Possible Answers:

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.

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:

Since the area of an equilateral triangle is solely dependent on the length of one side, it follows that the triangle with the greater sidelength has the greater area.

Statement 1 alone gives that the length of one side of  is greater than that on one side of .It follows that  has the greater area.

Statement 2 alone gives that , from which it follows that

and 

.

Again, this shows that  has the greater sidelength and the greater area.

Example Question #12 : Dsq: Calculating The Area Of An Equilateral Triangle

Give the area of equilateral triangle .

Statement 1:  is a diameter of a circle with circumference .

Statement 2:  is a side of a 45-45-90 triangle with area .

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:

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. To find the diameter of a circle with circumference , divide the circumference by  to get . This is also the length of each side of the triangle, so we can get the area using the area formula:

.

Assume Statement 2 alone. A 45-45-90 Triangle has congruent legs, and the area is half the product of their lengths, so if we let  be the common sidelength,

By the 45-45-90 Theorem, the hypotenuse has length  times this, or .

Since it is not given whether  is a leg or the hypotenuse of a right triangle, however, the length of  - and consequently, the area - is not clear.

Example Question #21 : Equilateral Triangles

Find the area of an equilateral triangle.

  1. A side measures .
  2. An angle measures .
Possible Answers:

Statement 1 alone is sufficient, but statement 2 alone is not sufficient to answer the question.

Each statement alone is sufficient to answer the question.

Statement 2 alone is sufficient, but statement 1 alone is not sufficient to answer the question.

Both statements taken together are sufficient to answer the question, but neither statement alone is sufficient.

Statements 1 and 2 are not sufficient, and additional data is needed to answer the question.

Correct answer:

Statement 1 alone is sufficient, but statement 2 alone is not sufficient to answer the question.

Explanation:

In an equilateral triangle all sides are of the same length and all internal angles measure to 

Statement 1: 

Where  represents the length of the side.

If we're given the side, we can calculate the area:

Statement 2: We don't need the angle to find the area.

Example Question #71 : Triangles

Consider equilateral triangle .

I) The area of triangle  is .

II) Side  is .

What is the height of ?

Possible Answers:

Neither statement is sufficient to solve the question. More information is needed.

Statement 1 is sufficient to solve the question, but statement 2 is not sufficient to solve the question.

Statement 2 is sufficient to solve the question, but statement 1 is not sufficient to solve the question.

Both statements taken together are sufficient to solve the question.

Each statement alone is enough to solve the question.

Correct answer:

Each statement alone is enough to solve the question.

Explanation:

Since is states that we are working with a equilateral triangle we can use the formula for area:

 where  is the side length. Once we have calculated the side length we can then plug that value along with the area into the equation:

 and solve for h.

Consider that equilateral triangles have equal sides. This means we can make ABC into two smaller triangles with hypotenuse of 13 and base of 6.5. We can use that to find the height. We can find the height using statement II.

Therefore, both statements alone are sufficient to solve the question.

Example Question #1 : Dsq: Calculating The Height Of An Equilateral Triangle

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What is the length of the height of  ?

(1) ,  is the midpoint of 

(2) 

Possible Answers:

Both statements together are sufficient

Statement 1 alone is sufficient

Statements 1 and 2 taken together are not sufficient

Statement 2 alone is sufficient

Each statement alone is sufficient

Correct answer:

Both statements together are sufficient

Explanation:

Firslty, we would need to have the length of the other sides of the triangles to calculate the height. Information about the angles could also be able to see whether the triangle is a special triangle.

From statement one we can say that triangle ABC is an equilateral triangle, since D is the mid point of the the basis. Moreover knowing , we can see that angle  is 60 degrees. Since D, the basis of the height is the midpoint it follows that  is also 60 degrees. Therefore  is also 60 degrees. Hence the triangle is equilateral. However, we don't know the length of any of the side.

 

Statement 2 gives us the piece of missing information. And alone statement 2 doesn't help us find the height.

It follows that both statements together are sufficient.

Example Question #3 : Dsq: Calculating The Height Of An Equilateral Triangle

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The equilateral triangle is inscribed in the circle. What is the length of the height?

(1) The center of the circle is at  of the vertices A, B and C.

(2) .

Possible Answers:

Statement 2 alone is sufficient.

Both statements together are sufficient.

Statements 1 and 2 taken together are not sufficient.

Each statement alone is sufficient.

Statement 1 alone is sufficient.

Correct answer:

Statements 1 and 2 taken together are not sufficient.

Explanation:

To be able to answer the question, we would need information about the radius or about the sides of the triangle.

Statement 1 tells us that the center of the circle is at   of the vertice. However this is a property and it will be the same in any equilateral triangle inscribed in a circle, indeed, the heights, whose intersection is the center of gravity, all intersect at  of the vertices.

Statement 2 also tells us something that we could have known from the properties of equilateral triangles. Indeed, equilateral triangles have all their 3 angles equal to .

Even by taking both statements together, we can't tell anything about the lengths of the height. Therefore the statements are insufficient.

Example Question #4 : Dsq: Calculating The Height Of An Equilateral Triangle

Consider the equilateral .

I) Side .

II)   has an area of .

What is the height of ?

Possible Answers:

Neither statement is sufficient to answer the question. More information is needed.

Statement I is sufficient to answer the question, but statement II is not sufficient to answer the question.

Statement II is sufficient to answer the question, but statement I is not sufficient to answer the question.

Both statements are needed to answer the question.

Either statement is sufficient to answer the question.

Correct answer:

Either statement is sufficient to answer the question.

Explanation:

I) Gives us the length of side w. Since this is an equilateral triangle, we really are given all three sides. From here we can break WHY into two smaller triangles and use either Pythagorean Theorem (or 30/60/90 triangle ratios) to find the height.

II) Gives us the area of WHY. If we recognize the fact that we can make two smaller 30/60/90 triangles from WHY, then we can make an equation with one variable to find the height.

Solve the following for b:

Thus, either statement is sufficient to answer the question.

Example Question #5 : Dsq: Calculating The Height Of An Equilateral Triangle

 is an equilateral triangle. An altitude of  is constructed from  to a point  on .

What is the length of ?

Statement 1:  has perimeter 36.

Statement 2:  has area .

Possible Answers:

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

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.

Correct answer:

EITHER statement ALONE is sufficient to answer the question.

Explanation:

From either statement alone, it is possible to find the length of one side of ; from Statement 1 alone, the perimeter 36 can be divided by 3 to yield side length 12, and from Statement 2 alone, the area formula for an equilateral triangle can be applied as follows:

Once this is found, the length of altitude  can be found by noting that this divides the triangle into two congruent 30-60-90 triangles and by applying the 30-60-90 Theorem:

and

Example Question #5 : Dsq: Calculating The Height Of An Equilateral Triangle

Given equilateral triangles  and , construct the altitude from  to  on , and the altitude from  to  on .

Which, if either, is longer,  or ?

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.

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:

EITHER statement ALONE is sufficient to answer the question.

Explanation:

Assume Statement 1 alone. If altitude  of  is constructed, the right triangle  is constructed as a consequence.  is a leg and  the hypotenuse of , so . Since by Statement 1, it is given that , then by substitution, , so  is the longer altitude.

Assume Statement 2 alone. 

, so

 divides  into two 30-60-90 triangles, one of which is  with shorter leg  and hypotenuse , so by the 30-60-90 Theorem, 

Again,  and  is the longer altitude.

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