a exor b= a'b + ab'
dual of a exor b = (a'+b).(a+b') // remember dual does not change nature of variable
=a'.a + a'b' +ba+ bb' =a'b' +ab = a exnor b
now, (a exor b)' = (a'b +ab')'
=(a+b')(a'+b)= a'b' +ab =a exnor b
hence, exor is an orthogonal function whose dual and complement both are same i.e exnor