A function f : A --> B is invertible if and only if it is bijective.
That is, f is one-to-one and onto. If f fails to be any of one-to-one or onto, then f is not invertible.
Now Consider a function f : --> defined by f(x) = x2.
If will be one-to-one if for x y, f(x) f(y).
Now consider x = 1 and y = - 1. Here x y.
Then, f( 1) = 1 = f(-1).
Hence, f fails to be one-to-one.
Therefore, f is not invertible.
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