In geometry, an isosceles triangle is a triangle having two sides of same length. The two angles opposite come the equal sides room equal and are always acute. Assorted formulas forisosceles triangles are explained below. The two vital formulas for isosceles triangles space thearea that a triangle and also the perimeter that a triangle.

You are watching: Finding the third side of an isosceles triangle

What space the Isosceles triangles Formulas?

An isosceles triangle hastwo political parties of same length and also two equal sidesjoin in ~ the same angle come the basic i.e. The 3rd side. Thus, in an isosceles triangle, thealtitude is perpendicular native the crest which is usual to the equal sides. Suchspecial nature of the isosceles trianglehelp us to calculate its area as well as its altitude with the aid of the isosceles triangleformulas.


Isosceles Triangle Formulas

Area of one Isosceles Triangle:It is the an are occupied by thetriangle. Right here we have three recipe to discover the area the a triangle, based on the given parameters.

Area = 1/2× Base× Height

Area = \(\fracb2 \sqrta^2-\fracb^24\)(Here a is the equal side, and b is the basic of the triangle.)

Area = 1/2×abSinα(Here a and also b are the lengths of 2 sides andα is the angle in between these sides.)

Perimeter of one Isosceles Triangle: In one isosceles triangle, there space three sides:two equalsides and also one base. In bespeak to calculate the perimeter of one isosceles triangle, the expression 2a+ bis used,

P = 2a+ b

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

Altitude of an Isosceles Triangle: In one isosceles triangle, its height is the perpendicular street from itsvertex come its base. In stimulate to calculate the heightof an isosceles triangle, the expressionh = √(a2–b2/4) is used,

h = \(\sqrta^2-\fracb^24\)

Let us examine a couple of examples to more plainly understand the use of formulas for isosceles triangles.


Breakdown tough concepts through an easy visuals.
Math will certainly no longer be a tough subject, specifically when you know the principles through visualizations.

Book a complimentary Trial Class

Examples using Formulas because that Isosceles Triangles

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

Solution:Applying Pythagoras'theorem:

a2= (b/2)2+ h2

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

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

Answer: The area of one isosceles triangle is12 unit2.​​​​

Example 2:Find the lateral next of one isosceles triangle through an area that 20 unit2and a basic of 10 units?

Solution:Using theformula 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 also perimeter of an isosceles triangle if its 2 equal sides space of size 6 unitsand the 3rd side is 8 units.


Givena = b = 6 units, c= 8 units

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

Perimeter the the isosceles triangle,

P = 2×a + b

P = 2×6 + 8

= 20 units

Altitude that the isosceles triangle,

h = √(a2–b2/4)

h =√(62–82/4)

h =√(36−16)

h =√20units

Area that the isosceles triangle,

A =1/2×b×h


=√20/4 square units


FAQs onFormulas for Isosceles Triangles

What Is Isosceles Triangle Formula in Geometry?

In geometry, the isosceles triangle formulas are characterized as the formulas because that calculatingthe area and perimeter of one isoscelestriangle.

Area = 1/2× Base× HeightArea = \(\fracb2 \sqrta^2-\fracb^24\)Area = 1/2×abSinα

(Here a and b space the lengths of 2 sides andαis the angle between these sides.)

How come UseIsosceles Triangle Formula?

We can use the isoscelestriangle formulas as follows:

Step 1: inspect for the parameter(area, perimeter, or height)to be obtained or calculated.Step 2: determine the next of the isoscelestriangle and also put the worth in the required formula - area, perimeter, or height.

In case,area, perimeter, or altitude that the isoscelestriangle room given, friend can find the measure up of the next of the triangle by equating the offered values come the corresponding isoscelestriangle formula.

What Is 'a' in IsoscelesTriangle Formula?

In an isosceles triangle formula, it is in it area, perimeter, or altitude, 'a' describes the measure of the same sides the the isoscelestriangle.

See more: Who Is Laura Marano Married To, Laura Marano

Area = 1/2× Base× HeightArea = \(\fracb2 \sqrta^2-\fracb^24\)Area = 1/2×abSinα

(Here a and b space the lengths of two sides andαis the angle between these sides.)

How To find Perimeterof Triangle using Isosceles Triangle Formula?

We know that the perimeter of any figureis the amount of all its sidesthus,

Step 1: identify the political parties of the isosceles triangle - two equal sides a and also base b.Step 2: put the worths in the perimeter formula, p = 2a+ bStep 3: write the worth so derived with an suitable unit.