Let ρ = s 1 + c , {\displaystyle \rho ={\frac {s}{1+c}},} so that ρ = tan(ϑ/2).
设 ρ = s 1 + c , {\displaystyle \rho ={\frac {s}{1+c}},} 所以 ρ = tan(ϑ/2)。