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`