Question

if f(a) subset of b then f is called​

Answer 1

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if f(a) subset of b then f is called a function.

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Answer 2

A function is said to be an into function from Set A to set B, if atleast one element of B is not connected to an element of A.

Here, when A is a proper subset of B, clearly, it will have lesser elements than set B, and those extra elements of set B will not be connected to any element of Set A. So it is an into function.