The definition of Isosceles triangle is given as “a triangle in which any two of its sides are equal in their measurements is called an isosceles triangle”. So if you are given a triangle and in that if two side’s lengths are same then we can conclude that the given triangle is isosceles triangle.
Important properties of isosceles triangle:
♦ The angles opposite to equal sides are equal in measurement.
In triangle PQR, the sides PQ and PR are equal in measurement so the opposite angles ∠Q and ∠R are equal.
♦ If any two angles are equal then their opposite sides are equal in measurement.
In above triangle DEF, ∠E and ∠F are equal so their opposite sides DE and DF are equal in measurement.
The above mentioned properties are very very important so I recommend you to remember this without forgetting.
Formula for Perimeter of isosceles triangle:
Let the triangle BCD be a isosceles triangle with BC = a units, BD = a units and CD = b units.
Then perimeter of triangle BCD = 2a + b units.
Hope that everything is understood without any doubts.