The closure of a variable is the variable itself, it means
cl(X) = X and cl(Y) = Y
so, if X ⊆ Y then
because cl(X) = X and cl(Y) = Y
put value in cl(X) ⊆ cl(Y )
we get, X⊆ Y.
Hence proof that
if X ⊆ Y then cl(X) ⊆ cl(Y ).
Exercise 4.5. Prove that for every sets X and Y in R"if X & Y then...
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I need help with 1.24
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