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Isosceles triangle
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===Area=== The area <math>T</math> of an isosceles triangle can be derived from the formula for its height, and from the general formula for the area of a triangle as half the product of base and height:{{sfnp|Harris|Stöcker|1998|page=78}} :<math>T=\frac{b}{4}\sqrt{4a^2-b^2}.</math> The same area formula can also be derived from [[Heron's formula]] for the area of a triangle from its three sides. However, applying Heron's formula directly can be [[numerically unstable]] for isosceles triangles with very sharp angles, because of the near-cancellation between the [[semiperimeter]] and side length in those triangles.{{sfnp|Kahan|2014}} If the apex angle <math>(\theta)</math> and leg lengths <math>(a)</math> of an isosceles triangle are known, then the area of that triangle is:{{sfnp|Young|2011|page=298}} :<math>T=\frac{1}{2}a^2\sin\theta.</math> This is a special case of the general formula for the area of a triangle as half the product of two sides times the sine of the included angle.{{sfnp|Young|2011|page=398}}
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