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Calculus: Differentiation and Its Applications

What Is Differentiation?

Differentiation is a fundamental concept in calculus that deals with the rate of change of a function. The derivative represents the slope or instantaneous rate of change at a specific point.

The Derivative: Basic Definition

The derivative of a function f(x) is defined as:

    \[ f'(x) = \lim_{\Delta x \to 0} \frac{f(x + \Delta x) - f(x)}{\Delta x} \]

This limit gives the slope of the tangent line to f(x) at point x.

Basic Differentiation Rules

Power Rule

If f(x) = x^n, then:

    \[ f'(x) = n \cdot x^{n-1} \]

Example:

    \[ \text{If } f(x) = x^3, \text{ then } f'(x) = 3x^2 \]

Constant Rule

If f(x) = c (where c is constant):

    \[ f'(x) = 0 \]

Sum and Difference Rule

For f(x) = g(x) \pm h(x):

    \[ f'(x) = g'(x) \pm h'(x) \]

Product Rule

For f(x) = u(x) \cdot v(x):

    \[ f'(x) = u'(x)v(x) + u(x)v'(x) \]

Quotient Rule

For f(x) = \frac{u(x)}{v(x)}:

    \[ f'(x) = \frac{v(x)u'(x) - u(x)v'(x)}{[v(x)]^2} \]

Applications of Differentiation

Finding Tangents to Curves

The derivative gives the slope of the tangent line at any point on a curve.

Example: Find the tangent to y = x^2 at x = 3.

  1. Differentiate: y' = 2x
  2. Evaluate at x = 3: y' = 6
  3. Equation: y - 9 = 6(x - 3) or y = 6x - 9

Velocity and Acceleration

For position s(t):

  • Velocity: v(t) = \frac{ds}{dt}
  • Acceleration: a(t) = \frac{dv}{dt} = \frac{d^2s}{dt^2}

Optimization Problems

Critical points (maxima/minima) occur where f'(x) = 0.

Example Problem

Problem: Find critical points of f(x) = 3x^4 - 4x^3 + 2x^2 - 5.

Solution:

  1. Differentiate:

        \[ f'(x) = 12x^3 - 12x^2 + 4x \]

  2. Set f'(x) = 0:

        \[ 4x(3x^2 - 3x + 1) = 0 \]

  3. Only real solution: x = 0

Common Mistakes

  1. Misapplying the Power Rule (e.g., forgetting to subtract 1 from the exponent).
  2. Confusing Product Rule with Sum Rule.
  3. Incorrectly applying the Quotient Rule numerator order.

Practice Questions

  1. Differentiate f(x) = 5x^3 + 2x^2 - x + 7.
  2. Find the tangent to y = x^3 - 4x^2 + 2x at x = 2.
  3. Given s(t) = t^2 - 4t + 5, find velocity at t = 3.

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