adv functions
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Show that tanx= (sinx/ cosx)
can be written as:
tan(xy) = (tanx  tany) / (1+ tanxtany)

Reiny
write tan (xy)
= sin(xy)/cos(xy)
= [sinxcosy  cosxsiny[/[cosxcosy + sinxsiny]
Now divide everybody by cosxcosy and it will all fall into place.
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