Which of the following functions are into functions if they are all defined from R to R?
If the range of a function is a proper subset of the co-domain, then the function is into.
Consider f(x)=sin x.
We know that the range of sin x is [−1,1].
Thus, range ⊂ co-domain.
i.e., There are elements in the co-domain that have no pre-images. Thus, the function is into.
Consider f(x)=cos x.
We know that the range of cos x is [−1,1]. Since range ⊂ co-domain, the function is into.
Consider f(x)=ex. This function ranges from (0,∞) and thus, range ⊂ co-domain. Therefore, the function is into.
Lastly, f(x)=x3. has R as its range and co-domain. Therefore, it is not an into function.