Let f : R → R be a function such that f ( x + y ) = f ( x ) + f ( y ) , ∀ x , y ∈ R . If f ( x ) is differentiable at x = 0 , then f ( x ) is differentiable only in a finite interval containing zero f ( x ) is continuous ∀ x ∈ R f ′ ( x ) is constant ∀ x ∈ R f ( x ) is differentiable except at finitely many points be obtained by heating?