# GMAT Math : Acute / Obtuse Triangles

## Example Questions

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

Which of the following is true of a triangle with sides that measure 15, 17, and 21?

It cannot exist.

It is a right triangle.

It is an obtuse triangle.

It is an acute triangle.

None of these statements can be proved without further information.

It is an acute triangle.

Explanation:

The triangle can exist by the Triangle Inequality, since the sum of the two smaller sides exceeds the greatest:

To determine whether the triangle is acute, right, or obtuse, add the squares of the two smaller sides, and compare the sum to the square of the largest side.

Since this sum is greater, the triangle is acute.

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

Let the three interior angles of a triangle measure , and . Which of the following statements is true about the triangle?

The triangle is isosceles and obtuse.

The triangle is isosceles and acute.

The triangle is scalene and acute.

The triangle is scalene and obtuse.

The triangle is scalene and right.

The triangle is scalene and obtuse.

Explanation:

If these are the measures of the interior angles of a triangle, then they total . Add the expressions, and solve for .

One angle measures  The others measure:

Since the largest angle measures greater than , the angle is obtuse, and the triangle is as well. Since the three angles each have different measure, their opposite sides do also, making the triangle scalene.

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

In  and . Which of the following values of  makes  a scalene triangle?

None of the other responses gives a correct answer.

Explanation:

The three sides of a scalene triangle have different measures, so 15 can be eliminated.

By the Triangle Inequality, the sum of the lengths of the two smaller sides must exceed the length of the third side. Since , 8 violates this theorem; since , 22 does as well.

10 is a valid measure of the third side, since ; it makes all three segments of different length, so it is the correct choice.

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

is a scalene triangle with perimeter 33; the length of each of its sides can be given by a prime whole number. What is the greatest possible length of its longest side?

This triangle cannot exist.

This triangle cannot exist.

Explanation:

By trial and error, we get four ways to add distinct primes to yield sum 33:

In each case, however the Triangle Inequality is violated - the sum of the two shortest lengths does not exceed the third.

No triangle can exist as described.

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

The lengths of the sides of a scalene triangle are all prime numbers, and so is the perimeter of the triangle. What is the least possible perimeter of the triangle?

Explanation:

A scalene triangle has three sides of different lengths, so we are looking for three distinct prime integers whose sum is a prime integer.

One of the sides cannot be 2, since the sum of 2 and two odd primes would be an even number greater than 2, a composite number. Therefore, beginning with the least three odd primes, add increasing triples of distinct prime numbers, as follows, until a solution presents itself:

- incorrect

- correct

The correct answer, 19, presents itself quickly.

### Example Question #8 : Calculating The Length Of The Side Of An Acute / Obtuse Triangle

is a scalene triangle with perimeter 47; the length of each of its sides can be given by a prime whole number. What is the greatest possible length of its longest side?

This triangle cannot exist.

Explanation:

A scalene triangle has three sides of different lengths, so we are looking for three distinct prime integers whose sum is 47.

There are ten ways to add three distinct primes to yield sum 47:

By the Triangle Inequality, the sum of the lengths of the shortest two sides must exceed that of the greatest. We can therefore eliminate all but four:

The greatest possible length of the longest side is 23.

### Example Question #9 : Calculating The Length Of The Side Of An Acute / Obtuse Triangle

is a scalene triangle with perimeter 30. . Which of the following cannot be equal to ?

Explanation:

The three sides of a scalene triangle have different measures. One measure  cannot have is 12, but this is not a choice.

It cannot be true that . Since the perimeter is

, we can find out what other value can be eliminated as follows:

Therefore, if , then , and the triangle is not scalene. 9 is the correct choice.

### Example Question #10 : Calculating The Length Of The Side Of An Acute / Obtuse Triangle

is an isosceles triangle with perimeter 43; the length of each of its sides can be given by a prime whole number. What is the greatest possible length of its longest side?

This triangle cannot exist.

Explanation:

We are looking for ways to add three primes to yield a sum of 43. Two or all three (since an equilateral triangle is considered isosceles) must be equal (although, since 43 is not a multiple of three, only two can be equal).

We will set the shared sidelength of the congruent sides to each prime number in turn up to 19:

By the Triangle Inequality, the sum of the lengths of the shortest two sides must exceed that of the greatest. We can therefore eliminate the first three. , and  include numbers that are not prime (21, 15, 9). This leaves us with only one possibility:

- greatest length 19

19 is the correct choice.

### Example Question #41 : Acute / Obtuse Triangles

is an equilateral triangle. Points  are the midpoints of , respectively.  is constructed.

All of the following are true except:

All of the statements in the other four choices are correct.

Each side of  is parallel to a side of .

The perimeter of  is twice that of .

The area of  is twice that of .

The area of  is twice that of .

Explanation:

The three sides of  are the midsegments of , so  is similar to .

By the Triangle Midsegment Theorem, each is parallel to one side of

By the same theorem, each has length exactly half of that side, giving  twice the perimeter of .

But since the sides of  have twice the length of those of , the area of , which varies directly as the square of a sidelength, must be four times that of .

The correct choice is the one that asserts that the area of  is twice that of .

### Example Question #42 : Acute / Obtuse Triangles

.

Order the angles of  from least to greatest measure.

The angles of  cannot be ordered from the information given.