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You only need elementary trigonometry to do this question if you use the guidance given in the hints.
London is at longitude $0^o$ and latitude $51.5^o$ North and Sydney at longitude $151^o$ East and latitude $34^o$ South. Taking the earth to be a sphere with radius 6378 kilometres, calculate the distance between London and Sydney.
If the flight path is the shortest route at an altitude of 6 kilometres calculate the distance along the flight path.