Acute triangle - Examples, Exercises and Solutions

Definition of Acute Triangle

An acute triangle has all acute angles, meaning each of its three angles measures less than 90° 90° degrees and the sum of all three together equals 180° 180° degrees. 

Practice Acute triangle

Exercise #1

What kid of triangle is given in the drawing?

90°90°90°AAABBBCCC

Video Solution

Step-by-Step Solution

The measure of angle C is 90°, therefore it is a right angle.

If one of the angles of the triangle is right, it is a right triangle.

Answer

Right triangle

Exercise #2

What kind of triangle is given in the drawing?

404040707070707070AAABBBCCC

Video Solution

Step-by-Step Solution

As all the angles of a triangle are less than 90° and the sum of the angles of a triangle equals 180°:

70+70+40=180 70+70+40=180

The triangle is isosceles.

Answer

Isosceles triangle

Exercise #3

What kid of triangle is the following

393939107107107343434AAABBBCCC

Video Solution

Step-by-Step Solution

Given that in an obtuse triangle it is enough for one of the angles to be greater than 90°, and in the given triangle we have an angle C greater than 90°,

C=107 C=107

Furthermore, the sum of the angles of the given triangle is 180 degrees so it is indeed a triangle:

107+34+39=180 107+34+39=180

The triangle is obtuse.

Answer

Obtuse Triangle

Exercise #4

What kind of triangle is given in the drawing?

999555999AAABBBCCC

Video Solution

Step-by-Step Solution

Given that sides AB and AC are both equal to 9, which means that the legs of the triangle are equal and the base BC is equal to 5,

Therefore, the triangle is isosceles.

Answer

Isosceles triangle

Exercise #5

Which kind of triangle is given in the drawing?

666666666AAABBBCCC

Video Solution

Step-by-Step Solution

As we know that sides AB, BC, and CA are all equal to 6,

All are equal to each other and, therefore, the triangle is equilateral.

Answer

Equilateral triangle

Exercise #1

What kind of triangle is given here?

111111555AAABBBCCC5.5

Video Solution

Step-by-Step Solution

Since none of the sides have the same length, it is a scalene triangle.

Answer

Scalene triangle

Exercise #2

What kind of triangle is the following

606060606060606060AAABBBCCC

Video Solution

Step-by-Step Solution

Since in the given triangle all angles are equal, all sides are also equal.

It is known that in an equilateral triangle the measure of the angles will always be equal to 60° since the sum of the angles in a triangle is 180 degrees:

60+60+60=180 60+60+60=180

Therefore, it is an equilateral triangle.

Answer

Equilateral triangle

Exercise #3

Given an equilateral triangle:

555

What is its perimeter?

Video Solution

Step-by-Step Solution

Since the triangle is equilateral, that is, all sides are equal to each other.

The perimeter of the triangle is equal to the sum of all sides together, the perimeter of the triangle in the drawing is equal to:

5+5+5=15 5+5+5=15

Answer

15

Exercise #4

Below is an equilateral triangle:

XXX

If the perimeter of the triangle is 33 cm, then what is the value of X?

Video Solution

Step-by-Step Solution

We know that in an equilateral triangle all sides are equal.

Therefore, if we know that one side is equal to X, then all sides are equal to X.

We know that the perimeter of the triangle is 33.

The perimeter of the triangle is equal to the sum of the sides together.

We replace the data:

x+x+x=33 x+x+x=33

3x=33 3x=33

We divide the two sections by 3:

3x3=333 \frac{3x}{3}=\frac{33}{3}

x=11 x=11

Answer

11

Exercise #5

Look at the isosceles triangle below:

444666

What is its perimeter?

Video Solution

Step-by-Step Solution

Since we are referring to an isosceles triangle, the two legs are equal to each other.

In the drawing, they give us the base which is equal to 4 and one side is equal to 6, therefore the other side is also equal to 6.

The perimeter of the triangle is equal to the sum of the sides and therefore:

6+6+4=12+4=16 6+6+4=12+4=16

Answer

16

Exercise #1

Given the isosceles triangle,

777121212

What is its perimeter?

Video Solution

Step-by-Step Solution

Since the triangle is isosceles, that means its two legs are equal to each other.

Therefore, the base is 7 and the other two sides are 12.

The perimeter of a triangle is equal to the sum of all sides together:

12+12+7=24+7=31 12+12+7=24+7=31

Answer

31

Exercise #2

Look at the isosceles triangle below:

5.65.65.6XXX

The perimeter of the triangle is 50.

What is the value of X?

Video Solution

Step-by-Step Solution

Since we know the triangle is isosceles, the other side will also be equal to X

Now we can replace the data to calculate X.

The perimeter of the triangle is equal to:

x+x+5.6=50 x+x+5.6=50

2x=505.6 2x=50-5.6

2x=44.4 2x=44.4

We divide both sides by 2:

2x2=44.42 \frac{2x}{2}=\frac{44.4}{2}

x=22.2 x=22.2

Answer

22.2

Exercise #3

Look at the following triangle:

2X2X2X3.5X3.5X3.5X3X3X3X

The perimeter of the triangle is 17.

What is the value of X?

Video Solution

Step-by-Step Solution

We know that the perimeter of a triangle is equal to the sum of all sides together, so we replace the data:

3x+2x+3.5x=17 3x+2x+3.5x=17

8.5x=17 8.5x=17

Divide the two sections by 8.5:

8.5x8.5=178.5 \frac{8.5x}{8.5}=\frac{17}{8.5}

x=2 x=2

Answer

2

Exercise #4

Triangle ABC is a right triangle.

The area of the triangle is 6 cm².

Calculate X and the length of the side BC.

S=6S=6S=6444X-1X-1X-1X+1X+1X+1AAACCCBBB

Video Solution

Step-by-Step Solution

We use the formula to calculate the area of the right triangle:

ACBC2=cateto×cateto2 \frac{AC\cdot BC}{2}=\frac{cateto\times cateto}{2}

And compare the expression with the area of the triangle 6 6

4(X1)2=6 \frac{4\cdot(X-1)}{2}=6

Multiplying the equation by the common denominator means that we multiply by 2 2

4(X1)=12 4(X-1)=12

We distribute the parentheses before the distributive property

4X4=12 4X-4=12 / +4 +4

4X=16 4X=16 / :4 :4

X=4 X=4

We replace X=4 X=4 in the expression BC BC and

find:

BC=X1=41=3 BC=X-1=4-1=3

Answer

X=4, BC=3

Exercise #5

Below is the Isosceles triangle ABC (AC = AB):

AAABBBCCCDDDEEE

In its interior, a line ED is drawn parallel to CB.

Is the triangle AED also an isosceles triangle?

Video Solution

Step-by-Step Solution

To demonstrate that triangle AED is isosceles, we must prove that its hypotenuses are equal or that the opposite angles to them are equal.

Given that angles ABC and ACB are equal (since they are equal opposite bisectors),

And since ED is parallel to BC, the angles ABC and ACB alternate and are equal to angles ADE and AED (alternate and equal angles between parallel lines)

Opposite angles ADE and AED are respectively sides AD and AE, and therefore are also equal (opposite equal angles, the legs of triangle AED are also equal)

Therefore, triangle ADE is isosceles.

Answer

AED isosceles

Topics learned in later sections

  1. Area
  2. The Sum of the Interior Angles of a Triangle
  3. The sides or edges of a triangle
  4. Triangle Height
  5. Exterior angles of a triangle
  6. Types of Triangles
  7. Obtuse Triangle
  8. Equilateral triangle
  9. Identification of an Isosceles Triangle
  10. Scalene triangle
  11. Isosceles triangle
  12. The Area of a Triangle
  13. Area of a right triangle
  14. Area of Isosceles Triangles
  15. Area of a Scalene Triangle
  16. Area of Equilateral Triangles
  17. Perimeter
  18. Triangle
  19. Perimeter of a triangle