An aircraft flies in the (x,z)-plane from the point (−a, 0) to the point (a, 0). (z = 0 is…

An aircraft flies in the (x,z)-plane from the point (−a, 0) to the point (a, 0). (z = 0 is ground level and the z-axis points vertically upwards.) The cost of flying the aircraft at height z is exp(−kz) per unit distance of flight, where k is a positive constant. Find the extremal for the problem of minimising the total cost of the journey. [Assume that ka