Công thức được xây dựng như sau 

$\dfrac{1}{S} = \sqrt {\left( {\dfrac{1}{{{h_1}}} + \dfrac{1}{{{h_2}}} + \dfrac{1}{{{h_3}}}} \right)\left( {\dfrac{1}{{{h_1}}} + \dfrac{1}{{{h_2}}} - \dfrac{1}{{{h_3}}}} \right)\left( {\dfrac{1}{{{h_1}}} - \dfrac{1}{{{h_2}}} + \dfrac{1}{{{h_3}}}} \right)\left( { - \dfrac{1}{{{h_1}}} + \dfrac{1}{{{h_2}}} + \dfrac{1}{{{h_3}}}} \right)} $