Drag/speed relationship

[tornadoflyer]

Hi,

in regard to why modern airlines fly at high altitude one book states "Since drag, in terms of speed, varies only with EAS, and since TAS in relation to EAS increases with altitude, it follows that the higher the altitude the higher the TAS for a given EAS and drag. Hence more miles are covered for the fuel expenditure necessary to produce the thrust to equal the drag". In reference to the drag equation, some books define velocity as TAS (some even use CAS). If TAS is the governing speed within the drag equation, how does this coincide with drag being only a function of EAS and not TAS?

Thanks in advance for any input.