Triangles are classified into different types on the basis of their sides and angles. Each of them has their own individual properties.

Equilateral triangle: A triangle whose all the three sides are equal.

Isosceles triangle: A triangle whose two sides are equal.

Scalene triangle: A triangle whose all three sides are unequal.

Acute angled triangle: A triangle whose all interior angles are less than 900.

Right angled triangle: A triangle whose one interior angle is 900.

Obtuse angled triangle: A triangle whose one interior angle is more than 900.

It has two sides of equal length. In the above figure, sides AB and AC are of equal length ‘a’ unit.

The angles opposite to equal sides are equal in measure. In the above figure, ∠ B and ∠C are of equal measure.

The altitude from vertex A to the base BC is the perpendicular bisector of the base BC.

The altitude from vertex A to the base BC is the angle bisector of the vertex angle ∠ A.

The altitude from the vertex divides an isosceles triangle into two congruent right-angled triangles. In the above figure, triangle ADB and triangle ADC are congruent right-angled triangles.

The median drawn from vertex A will bisect BC at right angles.

The medians drawn from vertex B and vertex C will not bisect the opposite sides AB and AC.

Two isosceles triangles are always similar.

In the right angled isosceles triangle, one angle is a right angle (90 degrees) and the other two angles are both 45 degrees.

In the right angled isosceles triangle, the altitude on the hypotenuse is half the length of the hypotenuse.

In the right angled isosceles triangle, the centre of circumcircle lies on the hypotenuse and the radius of circumcircle is half the length of hypotenuse.

An isosceles triangle whose two equal sides length is ‘a’ unit and length of its base is ’b’ unit. Then,

Solved Examples:

Q.1. Find the value of x in the given isosceles triangle ABC.

Solution: In the given isosceles ∆ABC, AB = AC.

Therefore, ∠ABC = ∠ACB = 75°. (Angle opposite to equal sides are equal)

Now, ∠ABC + ∠BAC + ∠ACB = 180°. (Interior angle sum property of a triangle)

⟹ 75° + x° + 75° = 180°

⟹ x° = 180° - 150°

⟹ x° = 30°

Q.2. Find the value of x in the given isosceles triangle PQR, whose base QR is extended to S.

Solution: In the given isosceles triangle PQR, PQ = PR.

Therefore, ∠PQR = ∠PRQ = x°. (Angle opposite to equal sides are equal)

Now, ∠PRQ + ∠PRS = 180°. (By linear pair)

⟹ x° + 120° = 180°.

⟹ x° = 180° - 120°.

⟹ x° = 60°.

Q.3. Find the perimeter and area of an isosceles triangle whose two equal sides and base length is 5 cm and 6 cm respectively.

Solution: Given, length of two equal sides of an isosceles triangle = a = 5 cm

And length of its base = b = 6 cm

Perimeter of an isosceles triangle = 2a + b

= 2(5) + 6

= 10 + 6 = 16 cm.

Q.4. If two angles of a triangle are 60° each, then the triangle is:

(a) Isosceles but not equilateral

(b) Scalene

(c) Equilateral

(d) Right-angled

Solution: (c)

By interior angle sum property of triangle,

The measure of the third angle of the given triangle comes out to be 60°.

So, each interior angle of a given triangle is 60°, which means each side of the triangle is equal (the sides opposite to equal angles are equal). Thus. The triangle is an equilateral triangle.

Q.5. If in an isosceles triangle, each of the base angles is 40°, then the triangle is:

(a) Right-angled triangle

(b) Acute angled triangle

(c) Obtuse angled triangle

(d) Isosceles right-angled triangle

Solution: (c)

By interior angle sum property of triangle,

The measure of the third angle of the given triangle comes out to be 120°.

Since, the measure of one of the angles of the triangle is greater than 90°. Therefore, the given triangle is an obtuse angled triangle.