tan(a+b)
=(tana+tanb)/(1-tanatanb)
=(2/5+3/7)/(1-2/5*3/7)
=(14/35+15/35)/(1-6/35)
=(29/35)/(29/35)
=1
tan(a-b)
=(tana-tanb)/(1-tanatanb)
=(2/5-3/7)/(1+2/5*3/7)
=(14/35-15/35)/(1+6/35)
=(-1/35)/(41/35)
=-1/41