tan^1的計算

設 α = tan⁻¹(x/5)，則 tanα = x/5
設 β = tan⁻¹(x/20)，則 tanβ = x/20

-90° - tan⁻¹(x/5) - tan⁻¹(x/20) = -180°
90° + tan⁻¹(x/5) + tan⁻¹(x/20) = 180°
90° + α + β = 180°
α + β = 90°
tan(α + β) = tan90°
(tanα + tanβ)/(1 - tanα tanβ) = ∞

故此分母 1 - tanα tan β ≈ 0
tanα tan β = 1
(x/5) (x/20) = 1
x²/100 = 1
x² = 100
x = 10 或 x = -10

若只取正值，x = 10