The equation you provided is a simplified form of an expression used in aircraft design and aerodynamics. It represents an approximation of the Oswald efficiency factor (e) for the entire aircraft based on the efficiency factors associated with different components of the aircraft, such as the wing, fuselage, and other components.
The equation is as follows:
1/e = 1/ewing + 1/efuselage + 1/eother
Where: e = Oswald efficiency factor for the entire aircraft ewing = Oswald efficiency factor for the wing efuselage = Oswald efficiency factor for the fuselage eother = Oswald efficiency factor for other components of the aircraft