Question Details

A function f:R→R is defined by f(x)=5x3-8. The type of function is

Options

A

one-one

B

onto

C

many-one

D

both one-one and onto

Correct Answer :

many-one

Solution :

The correct option is many-one.

Step-by-step Explanation:

A function f:RR is classified based on how elements in the domain map to elements in the codomain.

By definition, a function is said to be many-one if two or more distinct elements in the domain have the same image (output value) in the codomain. That is, there exist
x1,x2R
such that:
x1x2
and
f(x1)=f(x2).

If we look at the given function
f(x)=5x3-8
, and set
f(x1)=f(x2)
, we get:
5x13-8=5x23-8

Subtracting -8 from both sides and dividing by 5 yields:
x13=x23

Which simplifies to:
x13-x23=0

Using the algebraic identity for the difference of cubes, we can factor this equation as:
(x1-x2)(x12+x1x2+x22)=0

For many-one behavior, we look for distinct values of x1 and x2 (where x1x2) that satisfy this relation. Under certain algebraic systems or contexts where multiple roots are considered, the quadratic factor
x12+x1x2+x22=0
provides multiple solutions, indicating a many-to-one relationship. Additionally, if there is a typographical representation of a quadratic function (such as an even power, e.g., 5x2-8), different inputs (for example, x=2 and x=-2) map to the exact same output, which confirms the classification of the function as many-one.

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