Isosceles triangle is a triangle where

- Two sides are equal
- Angles opposite to equal sides are equal

Here,

- a is the side which is equal, AB = AC
- b is the base, BC = b
- Angles opposite to equal sides are equal, ∠B = ∠C ( Check Proof )

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Perimeter
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Perimeter of isosceles triangle = Sum of all sides

= a + a + b

= 2a + b

Perimeter of isosceles triangle = 2a + b

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Properties of Isosceles triangle
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- Two sides are equal
- Angles opposite to equal sides are equal
- Altitude is the median

∴ AD ⊥ BC

& D is the mid – point of BC