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I have been reading a textbook on cosmology and have come up against a self test question I just can't crack! Any help would be greatly appreciated:

"The Cosmological redshift of a quasar is normally deduced from its emission lines. Let this redshift be ze. Suppose the quasar has ejected a cloud of gas at velocity v with respect to the quasar. We detect the gas as absorption at redshift za. Show that the velocity of ejection is given by:

v/c = [ (1+ze)^2 - (1+za)^2 ] / [ (1+ze)^2 + (1+za)^2 ]

The "e" and "a" should be subscript, and the ^2 should be superscript (squared).

Thanks very much.

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# A redshift derivation question

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