I seek an elementary set theory proof. (All new to me.)
Let f:X→Y be a function. Ai are subsets of X, Bi are subsets of Y.
Prove that f(⋃iAi)=⋃if(Ai)
I sense that the approach is to show they're subsets of each other, but I can't out how to formulate this.
(Whilst we're at it, is the same true with the intersection?)
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