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Derivatives(I)

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Chapter Overview
In our previous studies, we learned how to analyze continuous and discontinuous functions. Now, we dive deeper by dividing continuous functions into two classes: derivable and non-derivable functions. This branch of mathematics is known as Differential Calculus, which allows us to measure how one quantity changes in relation to another. Developing a solid command of derivatives is essential for solving core mathematical, physical, and economic problems.
What this chapter covers
Increments & Rates of ChangeThe First Principles MethodFundamental Rules of DifferentiationThe Chain RuleParametric DifferentiationImplicit DifferentiationGeometrical Meaning of Derivatives
Learning Objectives
1
Explain the concept of increments and distinguish between average and instantaneous rates of change
2
Define the derivative of a function and derive standard formulas using the First Principles method
3
Apply basic differentiation rules including the Power, Product, Quotient, and Chain rules
4
Differentiate implicit equations and parametric functions by applying rules term-by-term
5
Interpret the derivative geometrically as the slope of a tangent line to a curve
The Slope of a Tangent
Key Insight
The derivative is not just a collection of algebraic rules. Geometrically, it represents the exact slope of the tangent line to a curve at any specific point. If you zoom in infinitely on a smooth curve, it looks like a straight line; the slope of that line is the derivative.
A vital rule to keep in mind: while continuity is a necessary condition for differentiability, it is not a sufficient one. A function can be continuous but fail to have a derivative at points where there is a sharp corner or spike, such as the vertex of $|x|$.
Key Definitions
Increment (\Delta x)
A small change in the value of a variable $x$. If $x$ changes from an initial value to a new value, the difference is the increment, which can be positive, negative, or zero.
Derivative
The limiting value of the ratio of the increment in the dependent variable to the increment in the independent variable, as the increment in the independent variable approaches zero.
Differentiation
The mathematical process of calculating the derivative or differential coefficient of a given function.
Differentiable Function
A function is differentiable at a given point if its derivative exists and is finite at that point.
Average Rate of Change
The ratio of the change in a function's output to the change in its input over a defined interval.
$$\frac{\Delta y}{\Delta x} = \frac{f(b) - f(a)}{b - a}$$
Instantaneous Rate of Change
The rate of change of a function at a single, exact instant, calculated by taking the limit of the average rate of change as the interval shrinks to zero.
$$\lim_{\Delta x \to 0} \frac{\Delta y}{\Delta x}$$
Average vs. Instantaneous Rate of Change
Understanding rates of change requires distinguishing between calculations done over an interval versus those done at a single point.
Average Rate of Change
$$\frac{\Delta y}{\Delta x} = \frac{f(x + \Delta x) - f(x)}{\Delta x}$$
Measures changes over a finite, measurable interval.
Geometrically represented by the slope of the secant line connecting two points on a curve.
Calculated using standard algebra without applying limits.
Example: Finding the average speed of a car over a 2-hour journey.
VS
Instantaneous Rate of Change
$$\frac{dy}{dx} = \lim_{\Delta x \to 0} \frac{f(x + \Delta x) - f(x)}{\Delta x}$$
Measures change at one specific, exact value.
Geometrically represented by the slope of the tangent line at a single point on a curve.
Requires evaluating limits as the interval width approaches zero.
Example: Checking the car's speedometer to see the speed at an exact second.
Memory Key: Secant line = Average rate. Tangent line = Instantaneous rate (Derivative).
Formula Sheet — Key Rules of Differentiation
The Power Rule
$$\frac{d}{dx}(x^n) = n x^{n-1}$$
Used when differentiating a variable raised to a constant power.
The Product Rule
$$\frac{d}{dx}(u \cdot v) = u \frac{dv}{dx} + v \frac{du}{dx}$$
Used when differentiating the product of two functions.
The Quotient Rule
$$\frac{d}{dx}\left(\frac{u}{v}\right) = \frac{v \frac{du}{dx} - u \frac{dv}{dx}}{v^2}$$
Used when differentiating a function divided by another function.
Choosing the Right Differentiation Rule
When you need to find a derivative, use this step-by-step decision pathway to select the correct rule.
Does the problem ask you to find the derivative 'from first principles'?
Yes
You must use the limit definition: lim (h->0) [f(x+h) - f(x)] / h.
No
Continue to check standard rules ↓
Is the equation a mixture of x and y terms that cannot be easily separated?
Yes
Use Implicit Differentiation. Differentiate term-by-term and attach dy/dx to y terms.
No
Continue to check standard rules ↓
Are x and y expressed separately in terms of a third variable 't' or 'θ'?
Yes
Use Parametric Differentiation: dy/dx = (dy/dt) / (dx/dt).
No
Continue to check standard rules ↓
Is the function composed of one function inside another, like (3x + 2)^5?
Yes
Use the Chain Rule. Differentiate the outer layer, then multiply by the derivative of the inner layer.
No
Use Basic Rules (Power, Product, or Quotient Rules).
Differentiation Memory Tricks
Trick: Simple phrasing makes remembering long formulas easier.
Steps to Remember:
  1. Product Rule Mnemonic: Repeat the phrase: 'First D-Second plus Second D-First'. This translates to: $u \cdot v' + v \cdot u'$.
  2. Quotient Rule Mnemonic: Learn this simple rhythm: 'Low D-High minus High D-Low, over the square of what's below'. This means: $\frac{\text{Low}(v) \cdot \text{D-High}(u') - \text{High}(u) \cdot \text{D-Low}(v')}{\text{Low}^2 (v^2)}$.
  3. The Chain Rule Onion Trick: Think of a nested function like an onion. You must peel it layer by layer, starting from the outermost function and working your way inside, multiplying the derivatives of each layer together.
Mnemonic: Low D-High minus High D-Low, over the square of what's below.
Common Mistakes to Avoid

Pay attention to these common pitfalls that often lead to lost marks in board exams.

  • Misunderstanding the Constant Rule: A common mistake is writing $\frac{d}{dx}(5) = 5$. Remember that the rate of change of any constant is zero, so $\frac{d}{dx}(5) = 0$.
  • Quotient Rule Sign Error: Mixing up the order in the numerator. It must be $v u' - u v'$. Writing $u v' - v u'$ is incorrect because subtraction is not commutative.
  • Neglecting the Inner Derivative in Chain Rule: When finding the derivative of $(3x+2)^2$, students often write $2(3x+2)^1$ and stop there. You must multiply by the derivative of the inside term ($3$), yielding $6(3x+2)$.
  • Directly using formulas in First Principles questions: If a question asks for the derivative from first principles, writing down the direct formula answer will get you zero marks. You must show the limit process.
NEB Exam Traps & Tips
NEB Exam Warning
Keep these targeted tips in mind to navigate tricky exam questions smoothly.
  • If you see a question involving square roots from first principles, like $\sqrt{ax+b}$, you will need to rationalize the numerator in your limit steps to resolve the indeterminate form.
  • When performing implicit differentiation on terms like $xy$, always remember to use the Product Rule. The derivative is $x \frac{dy}{dx} + y$, not just $x \frac{dy}{dx}$.
  • Before starting to differentiate a complex fraction, check if you can simplify it first. Factoring the algebraic terms beforehand can save you from a lengthy Quotient Rule calculation.
Question Recognition Guide
Keyword / Phrase in QuestionUse This
The question contains the phrase 'from first principles' or 'by definition'First Principles: Set up the limit $\lim_{h \to 0} \frac{f(x+h)-f(x)}{h}$ and solve.
An algebraic function is raised to a power, such as (ax + b)^nLinear Power Rule: Apply $n a (ax+b)^{n-1}$ or use the Chain Rule.
The equation contains both x and y terms mixed together, like x^2 + xy = y^3Implicit Differentiation: Differentiate term-by-term, applying the chain rule to y terms.
The variables x and y are given as separate equations of another variable 't' or 'θ'Parametric Differentiation: Calculate dx/dt and dy/dt separately, then divide.
The problem asks for the slope or equation of a tangent at a pointGeometrical Derivative: Find dy/dx, substitute the point's coordinates to find the slope m, and use $y - y_1 = m(x - x_1)$.
Geometric Meaning of the Derivative
This diagram illustrates how the average rate of change (secant line) becomes the instantaneous rate of change (tangent line). As the increment $\Delta x$ shrinks towards zero, point Q moves along the curve towards point P, and the blue secant line rotates to match the red tangent line.
P(x, y)Q(x+Δx, y+Δy)As Δx → 0, Secant (Blue) becomes Tangent (Red)
The transition of a secant line into a tangent line as the horizontal interval Δx approaches zero.
Solved Examples
1
Differentiate $y = 4x^5 + 7$ with respect to $x$.
Easy
2
Find $\frac{dy}{dx}$ for the function $y = (x^2 + 1)(3x - 2)$.
Medium
3
Find $\frac{dy}{dx}$ if $x^2 + y^2 = 25$.
Hard
MCQ Practice
Mixed Level — MCQ
1
The derivative of a constant k is:
Correct!
Incorrect. Correct: 0
2
If y = x^10, then the derivative y' is:
Correct!
Incorrect. Correct: 10x^9
3
Geometrically, the derivative dy/dx represents the:
Correct!
Incorrect. Correct: Slope of the tangent to the curve
4
Which of the following is the correct Product Rule formula for d/dx(uv)?
Correct!
Incorrect. Correct: uv' + vu'
5
Continuity is a _____ condition for derivability.
Correct!
Incorrect. Correct: Necessary
6
The derivative of 2x with respect to x is:
Correct!
Incorrect. Correct: 2
7
In parametric form, the derivative dy/dx is expressed as:
Correct!
Incorrect. Correct: (dy/dt) / (dx/dt)
8
The derivative of √x is:
Correct!
Incorrect. Correct: 1 / (2√x)
9
The process of calculating a derivative is called:
Correct!
Incorrect. Correct: Differentiation
10
If y = (2x + 3)^2, then dy/dx is:
Correct!
Incorrect. Correct: 4(2x + 3)
Short Questions (2 Marks Each)
1
Define the term 'increment'.
[2 marks]
2
State the Power Rule of differentiation.
[2 marks]
3
Find the derivative of $5x^2 - 3x$.
[2 marks]
4
Find $\frac{dy}{dx}$ if $x = t^2$ and $y = 2t$.
[2 marks]
5
Differentiate $y = \frac{1}{x}$ with respect to $x$.
[2 marks]
6
What is the derivative of $x^2$ at the point $x=3$?
[2 marks]
7
Find the slope of the curve $y=x^2$ at the origin $(0,0)$.
[2 marks]
8
Is every continuous function derivable? Explain briefly.
[2 marks]
9
Find the derivative of $(x+a)^n$ with respect to $x$, where $a$ is a constant.
[2 marks]
10
Find the derivative $y'$ for the function $y = \sqrt{4-x}$.
[2 marks]
Long Questions (Board Style)
1
Find the derivative of $x^3$ from first principles.
[3 marks]
2
Find the derivative of $\sqrt{x}$ from first principles.
[3 marks]
3
Find $\frac{dy}{dx}$ if $x^2 + xy + y^2 = 10$ using implicit differentiation.
[3 marks]
4
Differentiate $y = (3x^2 + 2)^4$ using the chain rule.
[3 marks]
5
Find $\frac{dy}{dx}$ for the parametric equations $x = a \cos^3 \theta$ and $y = a \sin^3 \theta$.
[3 marks]
6
Find the derivative of the function $y = \frac{x^2+1}{x^2-1}$ using the quotient rule.
[3 marks]
7
Differentiate $(1+x^2)^2$ with respect to $x^2$.
[3 marks]
8
Find the equation of the tangent to the curve $y = 2x^2 - 3x + 1$ at the point $(2, 3)$.
[3 marks]
9
Find the derivative of $y = e^{\sqrt{x}}$ using the chain rule.
[3 marks]
10
Find the derivative of $\frac{1}{x^2}$ from first principles.
[3 marks]
Past NEB Board Questions
Past NEB Board Questions
1
Find the derivative of $\sqrt{ax+b}$ from first principles.
NEB 2079 [3 marks]
2
Find $\frac{dy}{dx}$ if $x = at^2$ and $y = 2at$.
NEB 2078 [3 marks]
3
Find the derivative of the function $y = \frac{x^2+1}{x^2-1}$.
NEB 2077 [3 marks]
Chapter Test — Full Mixed Paper
Attempt this practice test in 45 minutes under quiet exam conditions.
1 Section A — MCQs (1 Mark Each)
Mixed Level — MCQ
1
The derivative of x^(1/2) is:
Correct!
Incorrect. Correct: 1 / (2√x)
2
The derivative of the constant 100 is:
Correct!
Incorrect. Correct: 0
3
The derivative of y = 3x is:
Correct!
Incorrect. Correct: 3
2 Section B — Short Questions (2 Marks Each)
1
Find the derivative of $y = (x^2+5)^3$.
[2 marks]
2
Find $\frac{dy}{dx}$ if $x=a\theta$ and $y=a/\theta$.
[2 marks]
3 Section C — Long Questions (3 Marks Each)
1
Find the derivative of $\sqrt{ax+b}$ from first principles.
[3 marks]
One-Page Revision Cheat Sheet
1
First Principles Limit: Always start with $\lim_{h \to 0} \frac{f(x+h)-f(x)}{h}$. Do not skip directly to short-cut formulas.
2
Power Rule: $\frac{d}{dx}(x^n) = n x^{n-1}$. Reduces power by 1 and moves exponent to the front.
3
Product Rule Mnemonic: First D-Second + Second D-First ($uv' + vu'$).
4
Quotient Rule Mnemonic: Low D-High minus High D-Low, over the square of what's below.
5
Chain Rule Onion Method: Differentiate the outer function, keeping the inside intact, then multiply by the derivative of the inside.
6
Implicit Differentiation: Treat $y$ as a function of $x$. Differentiate term-by-term and multiply by $\frac{dy}{dx}$ whenever differentiating $y$.
7
Parametric Differentiation: Calculate $\frac{dy}{dt}$ and $\frac{dx}{dt}$ separately, then divide to find $\frac{dy}{dx}$.
8
Geometrical Meaning: The derivative is the slope of the tangent line. Use the slope $m$ in $y - y_1 = m(x - x_1)$ to write the tangent equation.
9
Continuity vs. Differentiability: A function must be continuous to be differentiable. However, being continuous does not guarantee differentiability.
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