The approximation indication "≈" I take advantage of for decimal approximations with tilde "~" staying a rougher approximation. Whilst the algebraic formula for $b, c$ is not difficult to calculate, the algebraic formulation for $a$ is quite challenging to compute. Therefore the modern approach relies on numerical calculation of $a$. https://jeffreytneuk.ageeksblog.com/35096520/little-known-facts-about-por-qué-es-bueno-leer-el-tarot