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Question

Given a homomorphism `phi : A rightarrow B`, we define an equivalence relation ~ on A by: `x ~ y Leftrightarrow phi(x) = phi(y)`
`Rightarrow x - y in ker(phi)`. We can write that y = x + (y - x) = x + z , where z = y - x. Can we now conclude that `y - x in ker (phi) Rightarrow y in x + ker (phi)` ?