How to find the base of an isosceles triangle without the height

In geometry, an isosceles triangle is a triangle having two sides of equal length. The two angles opposite to the equal sides are equal and are always acute. Various formulas for isosceles triangles are explained below. The two important formulas for isosceles triangles are the area of a triangle and the perimeter of a triangle.

What Are the Isosceles Triangles Formulas?

An isosceles triangle has two sides of equal length and two equal sides join at the same angle to the base i.e. the third side. Thus, in an isosceles triangle, the altitude is perpendicular from the vertex which is common to the equal sides. Such special properties of the isosceles triangle help us to calculate its area as well as its altitude with the help of the isosceles triangle formulas. 

Isosceles Triangle Formulas

Area of an Isosceles Triangle: It is the space occupied by the triangle. Here we have three formulas to find the area of a triangle, based on the given parameters.

• Area = 1/2 × Base × Height

• Area = \(\frac{b}{2} \sqrt{a^{2}-\frac{b^{2}}{4}}\)
  (Here a is the equal side, and b is the base of the triangle.)

• Area = 1/2 ×abSinα
  (Here a and b are the lengths of two sides and α is the angle between these sides.)

Perimeter of an Isosceles Triangle: In an isosceles triangle, there are three sides: two equal sides and one base. In order to calculate the perimeter of an isosceles triangle, the expression 2a + b is used,

P = 2a + b

(Here, the length of the equal side is a and the length of the base is b)

Altitude of an Isosceles Triangle: In an isosceles triangle, its height is the perpendicular distance from its vertex to its base. In order to calculate the height of an isosceles triangle, the expression h = √(a2–b2/4) is used,

h = \(\sqrt{a^{2}-\frac{b^{2}}{4}}\)

Let us check a few examples to more clearly understand the use of formulas for isosceles triangles. 

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Examples Using Formulas for Isosceles Triangles

Example 1: Determine the area of an isosceles triangle that has a base 'b' of 8 units and the lateral side 'a' of 5 units?

Solution:  Applying Pythagoras' theorem:

a2 = (b/2)2 + h2

h2 = a2 - (b/2)2 = 52 - 42 which gives h = 3

Area 'A' = (1/2) × b × h = (1/2) 8 × 3 = 12 unit2

Answer: The area of an isosceles triangle is 12 unit2.​​​​

Example 2: Find the lateral side of an isosceles triangle with an area of 20 unit2 and a base of 10 units?

Solution: Using the formula of area of an isosceles triangle:

A = (1/2) b h = 20

Given b = 10,

To find: lateral side

h = 40 / 10 = 4

Applying Pythagora's theorem:

a2 = (b/2)2 + h2 = √ ( 52 + 42) = √41                                                                                                                                       

Answer: The lateral side of an isosceles triangle is √41.

Example 3: Calculate the area, altitude, and perimeter of an isosceles triangle if its two equal sides are of length 6 units and the third side is 8 units.

Solution:

Given a = b = 6 units, c = 8 units

To find: area, altitude, and perimeter of an isosceles triangle

Perimeter of the isosceles triangle,

P = 2×a + b

P = 2×6 + 8

= 20 units

Altitude of the isosceles triangle,

h = √(a2–b2/4)

h = √(62–82/4)

h = √(36−16)

h = √20 units

Area of the isosceles triangle,

A = 1/2×b×h

= 1/2×8×√20 

= √20/4 square units

Answer:

In geometry, the isosceles triangle formulas are defined as the formulas for calculating the area and perimeter of an isosceles triangle.

• Area = 1/2 × Base × Height
• Area = \(\frac{b}{2} \sqrt{a^{2}-\frac{b^{2}}{4}}\)
• Area = 1/2 ×abSinα

(Here a and b are the lengths of two sides and α is the angle between these sides.)

How To Use Isosceles Triangle Formula?

We can use the isosceles triangle formulas as follows:

• Step 1: Check for the parameter(area, perimeter, or height) to be derived or calculated.
• Step 2: Identify the side of the isosceles triangle and put the value in the required formula - area, perimeter, or height.

In case, area, perimeter, or altitude of the isosceles triangle are given, you can find the measure of the side of the triangle by equating the given values to the respective isosceles triangle formula.

What Is 'a' in Isosceles Triangle Formula?

In an isosceles triangle formula, be it area, perimeter, or altitude, 'a' refers to the measure of the equal sides of the isosceles triangle. 

• Area = 1/2 × Base × Height
• Area = \(\frac{b}{2} \sqrt{a^{2}-\frac{b^{2}}{4}}\)
• Area = 1/2 ×abSinα

(Here a and b are the lengths of two sides and α is the angle between these sides.)

How To Find Perimeter of Triangle Using Isosceles Triangle Formula?

We know that the perimeter of any figure is the sum of all its sides thus,

• Step 1: Identify the sides of the isosceles triangle - two equal sides a and base b.
• Step 2: Put the values in the perimeter formula, P = 2a + b 
• Step 3: Write the value so obtained with an appropriate unit.

An isosceles triangle is a triangle with two sides of the same length. These two equal sides always join at the same angle to the base (the third side), and meet directly above the midpoint of the base.[1] X Research source Go to source You can test this yourself with a ruler and two pencils of equal length: if you try to tilt the triangle to one direction or the other, you cannot get the tips of the pencils to meet. These special properties of the isosceles triangle allow you to calculate the area from just a couple pieces of information.

1. 1

   Review the area of a parallelogram. Squares and rectangles are parallelograms, as is any four-sided shape with two sets of parallel sides. All parallelograms have a simple area formula: area equals base multiplied by the height, or A = bh.[2] X Research source Go to source If you place the parallelogram flat on a horizontal surface, the base is the length of the side it is standing on. The height (as you would expect) is how high it is off the ground: the distance from the base to the opposite side. Always measure the height at a right (90 degree) angle to the base.

   • In squares and rectangles, the height is equal to the length of a vertical side, since these sides are at a right angle to the ground.

2. 2

   Compare triangles and parallelograms. There's a simple relationship between these two shapes. Cut any parallelogram in half along the diagonal, and it splits into two equal triangles. Similarly, if you have two identical triangles, you can always tape them together to make a parallelogram. This means that the area of any triangle can be written as A = ½bh, exactly half the size of a corresponding parallelogram.[3] X Research source Go to source

3. 3

   Find the isosceles triangle's base. Now you have the formula, but what exactly do "base" and "height" mean in an isosceles triangle? The base is the easy part: just use the third, unequal side of the isosceles.

   • For example, if your isosceles triangle has sides of 5 centimeters, 5 cm, and 6 cm, use 6 cm as the base.
   • If your triangle has three equal sides (equilateral), you can pick any one to be the base. An equilateral triangle is a special type of isosceles, but you can find its area the same way.[4] X Research source Go to source

4. 4

   Draw a line between the base to the opposite vertex. Make sure the line hits the base at a right angle. The length of this line is the height of your triangle, so label it h. Once you calculate the value of h, you'll be able to find the area.

   • In an isosceles triangle, this line will always hit the base at its exact midpoint.[5] X Research source Go to source

5. 5

   Look at one half of your isosceles triangle. Notice that the height line divided your isosceles triangle into two identical right triangles. Look at one of them and identify the three sides:

   • One of the short sides is equal to half the base:
     
     .
   • The other short side is the height, h.
   • The hypotenuse of the right triangle is one of the two equal sides of the isosceles. Let's call it s.

6. 6

   Set up the Pythagorean Theorem. Any time you know two sides of a right triangle and want to find the third, you can use the Pythagorean theorem:[6] X Research source Go to source (side 1)2 + (side 2)2 = (hypotenuse)2 Substitute the variables we're using for this problem to get

   
   .[7] X Research source Go to source
   • You probably learned the Pythagorean Theorem as
     
     . Writing it as "sides" and "hypotenuse" prevents confusion with your triangle's variables.

7. 7

   Solve for h. Remember, the area formula uses b and h, but you don't know the value of h yet. Rearrange the formula to solve for h:[8] X Research source Go to source

8. 8

   Plug in the values for your triangle to find h. Now that you know this formula, you can use it for any isosceles triangle where you know the sides. Just plug in the length of the base for b and the length of one of the equal sides for s, then calculate the value of h.

   • For example, you have an isosceles triangle with sides 5 cm, 5 cm, and 6 cm. b = 6 and s = 5.
   • Substitute these into your formula:
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     cm.

9. 9

   Plug the base and height into your area formula. Now you have what you need to use the formula from the start of this section: Area = ½bh. Just plug the values you found for b and h into this formula and calculate the answer. Remember to write your answer in terms of square units.[9] X Research source Go to source

   • To continue the example, the 5-5-6 triangle had a base of 6 cm and a height of 4 cm.
   • A = ½bhA = ½(6cm)(4cm)

     A = 12cm2.

10. 10

    Try a more difficult example. Most isosceles triangles are more difficult to work with than the last example. The height often contains a square root that doesn't simplify to an integer. If this happens, leave the height as a square root in simplest form. Here's an example:

    • What is the area of a triangle with sides 8 cm, 8 cm, and 4 cm?
    • Let the unequal side, 4 cm, be the base b.
    • The height
      
      
      
      
      
      
      
    • Simplify the square root by finding factors:
      
    • Area
      
      
      
      
      
      
      
    • Leave this answer as written, or enter it in a calculator to find a decimal estimate (about 15.49 square centimeters).

1. 1

   Start with a side and an angle. If you know some trigonometry, you can find the area of an isosceles triangle even if you don't know the length of one of its side. Here's an example problem where you only know the following:[10] X Research source Go to source

   • The length s of the two equal sides is 10 cm.
   • The angle θ between the two equal sides is 120 degrees.

2. 2

   Divide the isosceles into two right triangles. Draw a line down from the vertex between the two equal sides, that hits the base at a right angle. You now have two equal right triangles.[11] X Research source Go to source

   • This line divides θ perfectly in half. Each right triangle has an angle of ½θ, or in this case (½)(120) = 60 degrees.

3. 3

   Use trigonometry to find the value of h. Now that you have a right triangle, you can use the trigonometric functions sine, cosine, and tangent. In the example problem, you know the hypotenuse, and you want to find the value of h, the side adjacent to the known angle. Use the fact that cosine = adjacent / hypotenuse to solve for h:[12] X Research source Go to source

   • cos(θ/2) = h / s
   • cos(60º) = h / 10
   • h = 10cos(60º)

4. 4

   Find the value of the remaining side. There is one remaining unknown side of the right triangle, which you can call x. Solve for this using the definition sine = opposite / hypotenuse:

   • sin(θ/2) = x / s
   • sin(60º) = x / 10
   • x = 10sin(60º)

5. 5

   Relate x to the base of the isosceles triangle. You can now "zoom out" to the main isosceles triangle. Its total base b is equal to 2x, since it was divided into two segments each with a length of x.

6. 6

   Plug your values for h and b into the basic area formula. Now that you know the base and height, you can rely on the standard formula A = ½bh:

   • 
     
     
     
     
     
     
     
     
     
   • You can enter this into a calculator (set to degrees), which gives you an answer of about 43.3 square centimeters. Alternatively, use properties of trigonometry to simplify it to A = 50sin(120º).

7. 7

   Turn this into a universal formula. Now that you know how this is solved, you can rely on the general formula without going through the full process every time. Here's what you end up with if you repeat this process without using any specific values (and simplifying using properties of trigonometry):[13] X Research source Go to source

   • 
   • s is the length of one of the two equal sides.
   • θ is the angle between the two equal sides.

• Question

  How can I find the side of an isosceles triangle when only the area and the length of equal sides are given?

  A=area, L=length of 1 equal side, b=base, θ=HALF of angle between 2 equal sides. Split the triangle in half down the middle. The middle line is h, the height. Analyze the left triangle, where L is the hypotenuse and the smallest angle is θ. The smallest side is b/2, and the last side is h. sinθ = (b/2) / L --> b/2 = Lsinθ. cosθ = h/L --> h = Lcosθ. A = (1/2)bh = (b/2)h = (Lsinθ)(Lcosθ)=(L^2)sinθcosθ. sin(2θ) = 2sinθcosθ (by trig identities) --> sinθcosθ = (1/2)sin(2θ). --> A = (L^2)sinθcosθ = (1/2)(L^2)sin(2θ). Because A and L are known, the above equation can be used to find sin(2θ). Arcsin of sin(2θ) gives 2θ, allowing you to find θ. Then, you can find b from the equation: b/2 = Lsinθ.

• Question

  How can I show that a triangle is isoceles?

  Coordinate proof: Given the coordinates of the triangle's vertices, to prove that a triangle is isosceles plot the 3 points (optional). Use the distance formula to calculate the side length of each side of the triangle. If any two sides have equal side lengths, then the triangle is isosceles.

• Question

  How do I find the base of a triangle if there is no height and no area?

  You don't. You must be given certain information: perimeter, other sides, area, or height.

This article was co-authored by wikiHow Staff. Our trained team of editors and researchers validate articles for accuracy and comprehensiveness. wikiHow's Content Management Team carefully monitors the work from our editorial staff to ensure that each article is backed by trusted research and meets our high quality standards. This article has been viewed 641,538 times.

Co-authors: 21

Updated: September 13, 2022

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