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senx=3/5 --> cosx=-4/5 ; tgx=-3/4
cosy=12/13 --> seny= - 5/13 --> tang=-5/12
se(x+y)=senx·cosy + seny·cosx= 3/5·12/13 + 4/5·5/13 =56/65
cos(x+y)=cosx·cosy-senx·seny = -48/65 +15/65=-33/65
tg(x-y)=[tgx-tgy]/[1+tgx·tgy] = [-3/4+5/12]/[1+15/48] = -20/63