**A and C are identities.**

A. `tan(x-pi)=(tanx-tanpi)/(1+tanxtanpi)=(tanx)/1=tanx`

B. `cos(x+y)-cos(x-y)`

`=cosxcosy-sinxsiny-[cosxcosy+sinxsiny]`

`=-2sinxsiny`

C. `cos(x+y)+cos(x-y)`

`=cosxcosy-sinxsiny+cosxcosy+sinxsiny`

`=2cosxcosy`

D. `sin(x+y)+sin(x-y)`

`=sinxcosy+cosxsiny+sinxcosy-cosxsiny`

`=2sinxcosy`

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