JEE Advance - Mathematics (2012 - Paper 1 Offline - No. 19)

The function $$f:[0,3] \to [1,29]$$, defined by $$f(x) = 2{x^3} - 15{x^2} + 36x + 1$$, is
one-one and onto.
onto but not one-one.
one-one but not onto.
neither one-one nor onto.

Explanation

The function $$f:[0,3] \to [1,29]$$, defined by $$f(x) = 2x^3 - 15x^2 + 36x + 1$$, is :

First, calculate the derivative of $ f $ :

$$ f'(x) = 6x^2 - 30x + 36 $$

Which simplifies to :

$$ f'(x) = 6(x^2 - 5x + 6) = 6(x - 2)(x - 3) $$

IIT-JEE 2012 Paper 1 Offline Mathematics - Functions Question 10 English Explanation

For the given domain $[0, 3]$, the function $ f(x) $ is both increasing and decreasing. Therefore, $ f(x) $ is many-to-one.

Setting $ f'(x) = 0 $ gives the critical points :

$$ x = 2, 3 $$

We now evaluate the function at these points within the domain to determine the range :

$$ f(0) = 1 $$

$$ f(2) = 29 $$

$$ f(3) = 28 $$

Thus, the range of $ f $ is $[1, 29]$, indicating that the function is onto.

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