Equation Icon  ⌊ ( )2 ( ′ )2 ⌋ 2 d-ρ2 1- -1-f′2 −-b2 2 -1--- 1- ′′ -1-f-2 −-b2- 2 ds = 4 ρ2 + ρ ⌈f(x) 1 + 16 f ρ dτ + f(x) 1 + 4 f ρ − 16 f ρ dx ⌉ , (305 )