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Q1(i):
Find the maximum and minimum values, if any, of the following functions given by (i) $f(x) = (2x – 1)^2 + 3$

Solution :

Step 1: Identifying the Mathematical Structure

The given function is $f(x) = (2x - 1)^2 + 3$. This function represents a parabola opening upwards [Per the definition of a quadratic function $ax^2 + bx + c$ where $a > 0$].

Step 2: Analytical Derivation via Square Completion

We analyze the properties of the squared term $(2x - 1)^2$. For any real number $x$, the square of a real number is non-negative:

  • $(2x - 1)^2 \geq 0$ for all $x \in \mathbb{R}$ [By the property of squares in real numbers].
  • Adding $3$ to both sides of the inequality: $(2x - 1)^2 + 3 \geq 0 + 3$.
  • Therefore, $f(x) \geq 3$.

Step 3: Verification via First Derivative Test

To find the critical points, we compute the first derivative $f'(x)$ and set it to zero:

$f(x) = (2x - 1)^2 + 3$

$f'(x) = 2(2x - 1) \cdot \frac{d}{dx}(2x - 1)$ [By the Chain Rule of Differentiation]

$f'(x) = 2(2x - 1) \cdot 2 = 4(2x - 1)$

Set $f'(x) = 0$:

$4(2x - 1) = 0 \implies 2x = 1 \implies x = \frac{1}{2}$

Step 4: Second Derivative Test for Nature of Extremum

Find the second derivative $f''(x)$ to determine concavity:

$f''(x) = \frac{d}{dx}[4(2x - 1)] = 4 \cdot 2 = 8$

Since $f''(x) = 8 > 0$ for all $x$, the function is concave upwards at the critical point $x = \frac{1}{2}$ [By the Second Derivative Test]. Thus, the function attains a global minimum at $x = \frac{1}{2}$.

x y 1/2 (1/2, 3)

Step 5: Evaluating Minimum Value

Substitute $x = \frac{1}{2}$ back into the original function:

$f\left(\frac{1}{2}\right) = (2(\frac{1}{2}) - 1)^2 + 3$

$f\left(\frac{1}{2}\right) = (1 - 1)^2 + 3 = 0 + 3 = 3$

Since the parabola opens upwards and extends to infinity as $x \to \pm\infty$, there is no maximum value.

Final Solution: The function has a minimum value of $3$ at $x = \frac{1}{2}$, and it has no maximum value.


More Questions from Class 12 Mathematics Application of Derivatives EXERCISE 6.3


CBSE Solutions for Class 12 Mathematics Application of Derivatives


Chapters in CBSE - Class 12 Mathematics


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