I have inserted spaces. Consider f(x) belonging to f(X) this means : x belongs to X But X is a subset of Y So, x belongs to Y So, f(x) belongs to f(Y). Hence all f(x) in f(X) belong to f(Y) So, f(X) is a subset of f(Y) Example: f(x) = sin(x) : R -> R X = [0,2pi] Y = [-2pi,2pi] f(X) = [-1,1] f(Y) = [-1,1] So, f(X) = f(Y) Solution I have inserted spaces. Consider f(x) belonging to f(X) this means : x belongs to X But X is a subset of Y So, x belongs to Y So, f(x) belongs to f(Y). Hence all f(x) in f(X) belong to f(Y) So, f(X) is a subset of f(Y) Example: f(x) = sin(x) : R -> R X = [0,2pi] Y = [-2pi,2pi] f(X) = [-1,1] f(Y) = [-1,1] So, f(X) = f(Y).