GMAT Math : Calculating the height of an acute / obtuse triangle

Study concepts, example questions & explanations for GMAT Math

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Example Question #1 : Calculating The Height Of An Acute / Obtuse Triangle

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Triangle  has height . What is the length of , knowing that  and ?

Possible Answers:

Correct answer:

Explanation:

To solve this equation, we need to calculate the length of the height with the Pythagorean Theorem.

We could also recognize that since  and , the triangle is a Pythagorean Triple, in other words, its sides will be in ratio  where  is a constant.

Here  and therefore, the length of height BD must be , which is our final answer.

Example Question #2 : Calculating The Height Of An Acute / Obtuse Triangle

The largest angle of an obtuse isosceles triangle is  .  If two of the sides have an equal length of  ,  what is the height of the triangle?

Possible Answers:

Correct answer:

Explanation:

If the largest angle of the obtuse isosceles triangle is  , then this is the unique angle in between the two sides with an equal length of .  We can imagine that the height of this isosceles triangle is simply the third side of a triangle formed by half of its base and the length of either equal side. That is, if we bisected the    angle with a line perpendicular to the base of the obtuse isosceles triangle, this line would be the height of the triangle. If we bisected the    angle, we would have two congruent triangles with angles of   between the height and each side of equal length. This means the cosine of that angle will be equal to the length of the height over the length of either equal side, which gives us:

Example Question #3 : Calculating The Height Of An Acute / Obtuse Triangle

One angle of an obtuse isosceles triangle has a measure of  .  If the length of the two equivalent sides is  ,  what is the height of the triangle?

Possible Answers:

Correct answer:

Explanation:

If one measure of an obtuse isosceles triangle is  ,  then this is obviously the unique angle that classifies the triangle as obtuse, which tells us that this is the angle between the two sides with an equivalent length of  .  The height of the triangle is given by a line that bisects this angle.  This tells us that the angle between the height and the sides of equivalent length is  ,  and because we know the length of the equivalent sides we can solve for the height as follows, where    is the height of the triangle and    is the length of the equivalent sides:

Example Question #4 : Calculating The Height Of An Acute / Obtuse Triangle

Given:  with  and .

Construct the altitude of  from  to a point  on . What is the length of ?

Possible Answers:

Correct answer:

Explanation:

 is shown below, along with altitude .

Isosceles

By the Isosceles Triangle Theorem, since  is isosceles with . By the Hypotenuse-Leg Theorem, the altitude cuts  into congruent triangles  and , so ; this makes  the midpoint of  has length 42, so  measures half this, or 21.

Also, since , and , by definition, is perpendicular to  is a 30-60-90 triangle. By the 30-60-90 Triangle Theorem, , as the shorter leg of , has length equal to that of longer leg  divided by ; that is, 

Example Question #5 : Calculating The Height Of An Acute / Obtuse Triangle

Given:  with 

Construct the altitude of  from  to a point  on . What is the length of ?

Possible Answers:

Correct answer:

Explanation:

 is shown below, along with altitude .

Triangle_1

Since  is, by definition, perpendicular to , it divides the triangle into 45-45-90 triangle  and 30-60-90 triangle .

Let  be the length of . By the 45-45-90 Theorem,  and , the legs of , are congruent, so ; by the 30-60-90 Theorem, short leg  of  has as its length that of  divided by , or . Therefore, the length of  is:

We are given that , so 

We can simplify this by multiplying both numerator and denominator by , thereby rationalizing the denominator:

Example Question #6 : Calculating The Height Of An Acute / Obtuse Triangle

Given:  with 

Construct the altitude of  from  to a point  on . Between which two consecutive integers does the length of  fall?

Possible Answers:

Between 9 and 10

Between 5 and 6

Between 7 and 8

Between 6 and 7

Between 8 and 9

Correct answer:

Between 7 and 8

Explanation:

Construct two altitudes of the triangle, one from  to a point  on , and the one stated in the question. 

Isosceles_4

 is isosceles, so the median  cuts it into two congruent triangles;  is the midpoint, so (as marked above)  has length half that of , or half of 10, which is 5. By the Pythagorean Theorem,

The area of a triangle is one half the product of the length of any base and its corresponding height; this is , but it is also . Since we know all three sidelengths other than that of , we can find the length of the altitude by setting the two expressions equal to each other and solving for :

To find out what two integers this falls between, square it:

Since , it follows that .

Example Question #7 : Calculating The Height Of An Acute / Obtuse Triangle

Given:  with .

Construct the altitude of  from  to a point  on . What is the length of ?

Possible Answers:

Correct answer:

Explanation:

 is shown below, along with altitude .

Triangle_1

Since  is, by definition, perpendicular to , it divides the triangle into 45-45-90 triangle  and the 30-60-90 triangle .

Let  be the length of . By the 45-45-90 Theorem, , and , the legs of , are congruent, so ; by the 30-60-90 Theorem, long leg  of  has length  times that of , or . Therefore, the length of  is:

We are given that , so 

and 

We can simplify this by multiplying both numerator and denominator by , thereby rationalizing the denominator:

 

Example Question #132 : Triangles

Given:  with , construct three altitudes of  - one from  to a point  on , another from  to a point  on , and a third from  to a point  on . Order the altitudes, , and  from shortest to longest.

Possible Answers:

Correct answer:

Explanation:

The area  of a triangle is half the product of the lengths of a base and that of its corresponding altitude. If we let  and  (height) stand for those lengths, respectively, the formula is

,

which can be restated as:

It follows that in the same triangle, the length of an altitude is inversely proportional to the length of the corresponding base, so the longest base will correspond to the shortest altitude, and vice versa.

Since, in descending order by length, the sides of the triangle are

,

their corresponding altitudes are, in ascending order by length,

.

Example Question #133 : Triangles

Given:  with 

Construct two altitudes of : one from  to a point  on , and the other from  to a point  on . Give the ratio of the length of  to that of .

Possible Answers:

Correct answer:

Explanation:

 is shown below, along with altitudes  and ; note that  has been extended to a ray  to facilitate the location of the point 

Isosceles_3

For the sake of simplicity, we will call the measure of  1; the ratio is the same regarless of the actual measure, and the measure of  willl give us the desired ratio. 

Since , and , by definition, is perpendicular to  is a 30-60-90 triangle. By the 30-60-90 Theorem, hypotenuse  of  has length twice that of short leg , so .

Since an exterior angle of a triangle has as its measure the sum of those of its remote interior angles, 

.

By defintiion of an altitude,  is perpendicular to , making  a 30-60-90 triangle. By the 30-60-90 Theorem, shorter leg  of  has half the length of hypotenuse , so ; also, longer leg  has length  times this, or .

The correct choice is therefore that the ratio of the lengths is .

 

Example Question #7 : Calculating The Height Of An Acute / Obtuse Triangle

Given:  with  and .

Construct the altitude of  from  to a point  on . What is the length of ?

Possible Answers:

Correct answer:

Explanation:

 is shown below, along with altitude ; note that  has been extended to a ray  to facilitate the location of the point 

Isosceles_2

Since an exterior angle of a triangle has as its measure the sum of those of its remote interior angles, 

By definition of an altitude,  is perpendicular to , making  a right triangle and  a 30-60-90 triangle. By the 30-60-90 Triangle Theorem, shorter leg  of  has half the length of hypotenuse —that is, half of 48, or 24; longer leg  has length  times this, or , which is the correct choice.

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