GMAT Math : DSQ: Calculating the height of an equilateral triangle

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

Consider equilateral triangle .

I) The area of triangle  is .

II) Side  is .

What is the height of ?

Possible Answers:

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

Each statement alone is enough to solve the question.

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

Both statements taken together are sufficient to solve the question.

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

Statement 1 alone is sufficient

Statement 2 alone is sufficient

Each statement alone is sufficient

Statements 1 and 2 taken together are not sufficient

Both statements together are 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 #2 : 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:

Both statements together are sufficient.

Statements 1 and 2 taken together are not sufficient.

Statement 1 alone is sufficient.

Statement 2 alone is sufficient.

Each statement 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:

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

Either statement is sufficient to answer the question.

Both statements are needed to answer the question.

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

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

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 #3 : 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.

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. 

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 #4 : 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:

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.

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:

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.

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 .

True or false: 

Statement 1: A circle of area less than  can be inscribed inside 

Statement 2:  is a chord of a circle of area .

Possible Answers:

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. 

EITHER statement ALONE is sufficient to answer the question.

Correct answer:

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

Explanation:

Assume Statement 1 alone. The inscribed circle, or "incircle," of a triangle has as its center the mutual point of intersection of the bisectors of the three angles, which, in the case of an equilateral triangle, coincide with the altitudes. , its three altitudes, and the incircle are shown below:

 Incircle

If the area of the incircle is less than , then the upper bound of the radius, which is , can be found as follows:

and  has length less than 4. Also, the point of intersection of the three altitudes divides each altitude into two segments, the ratio of whose lengths is 2 to 1, so 

and 

Therefore, Statement 1 only tells us that , leaving open the possibility that  may be less than, equal to or greater than 10.

Assume Statement 2 alone. The radius of a circle of area  can be found as follows:

The diameter of the circle is twice this, or . Since the longest chords of a circle are its diameters, then any chord in this circle must have length less than or equal to this. Statement 2 tells us that

Now examine the above diagram. , as half of an equilateral triangle, is a 30-60-90 triangle, so by the 30-60-90 Triangle Theorem, 

and 

 is therefore a true statement.

Example Question #6 : 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:  is inscribed inside a circle of circumference .

Statement 2:  is a chord of a circle of area .

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.

EITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are insufficient 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. The circumscribed circle, or "circumcircle," of a triangle has as its center the mutual point of intersection of the perpendicular bisectors of the three sides, which, in the case of an equilateral triangle, coincide with the altitudes. , its three altitudes, and the circumcircle are shown below:

Circumcircle

The circle has circumference , so its radius, which is equal to the length of , can be found by dividing this by  to yield

.

Also, the point of intersection of the three altitudes divides each altitude into two segments, the ratio of whose lengths is 2 to 1, so 

.

Assume Statement 2 alone. The radius of the circle can be found using the area formula for the circle, and the diameter can be found by doubling this. This diameter, however, only provides an upper bound for the length of a chord of the circle; if  is a chord of this circle, its length cannot be determined, only a range in which its length must fall. Therefore, Statement 2 is insufficient.

Example Question #7 : Dsq: Calculating The Height Of An Equilateral Triangle

Given  and , with  an equilateral triangle. Construct the altitude from  to  on , and the altitude from  to  on .

Which, if either, of   and  is longer? 

Statement 1: 

Statement 2:  is a right angle.

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. 

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:

Assume both statements are true. From Statement 1 alone,  , and , so  and . Therefore, between  and , two pairs of corresponding sides are congruent. 

 is an equilateral triangle, so ; from Statement 2,  is a right angle, so . This means that the included angle in  is of greater measure, so by the Side-Angle-Side Inequality Theorem, or Hinge Theorem, it has the longer opposite side, or . Both triangles are isosceles, so both altitudes divide the triangles into congruent right triangles, and by congruence,  and  are the midpoints of their respective sides. This means that 

By the Pythagorean Theorem,

 

and 

Since  and ,

meaning that  is the longer altitude.

Note that this depended on knowing both statements to be true. Statement 1 alone is insufficient, since, for example, had  measured less than , then by the same reasoning,  would have been the shorter altitude. Statement 2 alone is insufficient because it gives information only about one angle, and nothing about any side lengths.

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

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

True or false:  or  have the same length.

Statement 1:  and  are chords of the same circle.

Statement 2:   and  have the same 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 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.

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

Correct answer:

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

Explanation:

Statement 1 alone is inconclusive, since chords of the same circle can have different lengths.

Statement 2 alone is conclusive. The common side length  of an equilateral triangle depends solely on the area, so it follows that the sides of two triangles of equal area will have the same common side length. Also, each altitude divides its triangle into two 30-60-90 triangles. Examining  and , we can easily find that these triangles are congruent by way of the Angle-Side-Angle. Postulate, so it follows by triangle congruence that .

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