Explanation vs Derivation Examples: 46 Problems with Answers

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Explanation vs Derivation.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Math.

Concept Recap

The distinction between explaining WHY a result is true (conceptual insight) and showing HOW it can be derived step by step (procedural derivation).

Derivation: here are the steps. Explanation: here's why it makes sense.

Read the full concept explanation →

How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Explanation gives the conceptual reason a result is true; derivation gives the verifiable step-by-step path that produces it.

Common stuck point: The procedure for explanation vs derivation is the easy part; the trap is handing in a derivation when asked to explain. Asking "Am I being asked to make the result feel reasonable, or to produce it through verifiable steps?" first is what keeps a correct-looking calculation from being attached to the wrong concept.

Sense of Study hint: Ask: Am I being asked to make the result feel reasonable, or to produce it through verifiable steps?

Worked Examples

Example 1

easy

Derive the quadratic formula from

ax^2+bx+c=0

, then give an explanation of what the formula tells us conceptually.

Answer

x = \frac{- b \pm \sqrt{b^{2} - 4 a c}}{2 a}

First step

Derivation — start from

ax^2+bx+c=0

and complete the square. Divide by

a

x^2 + \frac{b}{a}x = -\frac{c}{a}

. Add

\left(\frac{b}{2a}\right)^2

to both sides:

\left(x+\frac{b}{2a}\right)^2 = \frac{b^2-4ac}{4a^2}

Full solution

2
Take square roots of both sides: $x + \frac{b}{2a} = \pm\frac{\sqrt{b^2-4ac}}{2a}$ . Solve for $x$ : $x = \frac{-b \pm \sqrt{b^2-4ac}}{2a}$ .
3
Conceptual explanation: the formula locates each root at signed distance $\frac{\sqrt{b^2-4ac}}{2a}$ from the axis of symmetry $x = -\frac{b}{2a}$ . The discriminant $\Delta = b^2-4ac$ signals: $\Delta > 0$ gives two distinct real roots, $\Delta = 0$ gives one repeated root, $\Delta < 0$ gives two complex-conjugate roots.

A derivation shows how a result follows step-by-step from axioms or earlier results. An explanation tells you what the result means and why it behaves as it does. Both are essential: derivation establishes validity, explanation builds understanding.

Example 2

medium

The sum formula

\sum_{k=1}^{n}k = \frac{n(n+1)}{2}

can be derived by induction or explained by Gauss's pairing argument. Give both.

Example 3

medium

Derive that the sum of the first

n

odd numbers equals

n^2

. Then give a visual explanation using L-shaped 'gnomons'.

Example 4

medium

Derive

\binom{n}{k} = \frac{n!}{k!(n-k)!}

from the definition. Then explain WHY the symmetry

\binom{n}{k} = \binom{n}{n-k}

holds.

Example 5

medium

Derive

\ln(ab) = \ln a + \ln b

from the definition

\ln x = \int_1^x \tfrac{1}{t}\,dt

. Then explain why a logarithm 'converts multiplication into addition'.

Example 6

hard

Derive Euler's formula

e^{i\theta} = \cos\theta + i\sin\theta

via Taylor series. Then give a one-sentence explanation involving rotation.

Example 7

hard

Derive the formula

\sum_{k=0}^{n} \binom{n}{k} = 2^n

. Then explain the combinatorial WHY.

Example 8

hard

Derive that

e = \lim_{n \to \infty}(1 + 1/n)^n

. Explain what this means about compound interest.

Example 9

challenge

Derive

e^{i\pi} + 1 = 0

from Euler's formula. Explain its conceptual content beyond the algebraic manipulation.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy

You know that

(a+b)^2 = a^2+2ab+b^2

. Give an explanation (not just algebraic expansion) of why the cross-term

2ab

appears.

Example 2

medium

State the difference between a derivation and an explanation in mathematics, then give one example of each for the statement

e^{i\pi}+1=0

Example 3

easy

The derivation of

(a+b)^2=a^2+2ab+b^2

uses FOIL. The explanation is a square split into regions. How many regions does the geometric picture have? Give the count.

Example 4

easy

Why is

a^0=1

? The explanation: dividing

a^n/a^n

. What does

a^n/a^n

equal for any nonzero

a

Example 5

easy

The quadratic formula is derived by completing the square. The explanation is 'every quadratic is a shifted parabola.' How many real roots does

x^2+1=0

have?

Example 6

easy

Why does multiplying two negatives give a positive? Explanation via patterns:

-1\times-1

equals what?

Example 7

easy

A derivation shows

0.999\ldots=1

10x-x=9

. The explanation: no number fits between them. What is

9x

x=0.999\ldots

Example 8

easy

The derivation of the circle area uses integration; the explanation unrolls it into a triangle of base

2\pi r

and height

r

. What is that triangle's area?

Example 9

easy

Why does the sum of interior angles of a triangle equal

180°

? Explanation: tearing the corners and lining them up forms a straight line. How many degrees is a straight line?

Example 10

easy

A formula

d=\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}

is derived from Pythagoras. For points

(0,0)

and

(3,4)

, give

d

Example 11

medium

The derivation of

1+2+\cdots+n=\frac{n(n+1)}{2}

uses pairing; the explanation is a staircase doubling into a rectangle. For

n=6

, give the rectangle's area (which is

n(n+1)

Example 12

medium

The derivation of the derivative of

x^2

uses limits; the explanation is 'slope of the tangent.' Give

\frac{d}{dx}x^2

x=3

Example 13

medium

Why does

\frac{a}{b}\div\frac{c}{d}=\frac{a}{b}\cdot\frac{d}{c}

? Explanation: dividing by a fraction asks 'how many fit.' Compute

\frac{1}{2}\div\frac{1}{4}

Example 14

medium

The derivation of

\sin^2\theta+\cos^2\theta=1

uses the unit circle. For

\theta

with

\cos\theta=0.6

, give

\sin^2\theta

Example 15

medium

A short derivation can hide insight. The product rule

(fg)'=f'g+fg'

is explained by area-change of a rectangle. If

f=g=x

x=2

, give

(fg)'=(x^2)'

there.

Example 16

medium

Why is the median sometimes better than the mean? Explanation: it resists outliers. For data

1,2,3,4,100

, give the median.

Example 17

challenge

Two derivations of

\binom{n}{k}=\binom{n}{n-k}

: algebraic (factorials) and combinatorial (choosing

k

to include = choosing

n-k

to exclude). For

n=7,k=2

, give

\binom{7}{2}

Example 18

challenge

The formula

\sum_{i=1}^{n} i^2=\frac{n(n+1)(2n+1)}{6}

has a derivation by telescoping and an explanation via stacking layers. Compute it for

n=3

Example 19

challenge

Why does Gaussian elimination work? Explanation: row operations preserve the solution set. Solving

\begin{cases}x+y=5\\x-y=1\end{cases}

, give

x

Example 20

medium

Why does an odd function satisfy

\int_{-a}^{a} f=0

? Explanation: symmetric areas cancel. For

f(x)=x

[-2,2]

, give the integral.

Example 21

medium

Why is

\frac{d}{dx}\sin x=\cos x

? Explanation: slope of sine matches cosine's height. At

x=0

, give

\cos 0

Example 22

medium

A derivation can be opaque. Why is

\sum_{i=1}^{n}(2i-1)=n^2

? Explanation: nested L-shapes build a square. Give the value for

n=4

Example 23

easy

The area of a triangle is

\tfrac{1}{2} b h

. Give the geometric explanation (NOT a formal derivation) for the

\tfrac{1}{2}

Example 24

easy

Why is

a^0 = 1

for nonzero

a

? Give a pattern-based explanation, not a derivation.

Example 25

easy

Derive

(a + b)(a - b) = a^2 - b^2

by expanding. The geometric explanation: subtract a small square from a big one. How many shaded rectangles appear in the picture proof?

Example 26

easy

Why does adding

0

to any number leave it unchanged? Give the explanation in plain language.

Example 27

easy

Geometric explanation: the circumference of a circle is

2\pi r

. What does the

2\pi

represent?

Example 28

medium

Explain WHY the slope of a line through

(x_1, y_1)

and

(x_2, y_2)

\frac{y_2 - y_1}{x_2 - x_1}

, not just derive it.

Example 29

medium

The Pythagorean theorem is

a^2 + b^2 = c^2

. Give a one-sentence explanation of why squares appear, not just a derivation.

Example 30

medium

Derive the formula for the sum of a geometric series

S = a + ar + ar^2 + \cdots + ar^{n-1}

. Then explain the trick of multiplying by

r

Example 31

medium

Why is the derivative of

x^2

equal to

2x

? Give a geometric/algebraic explanation, not just the power rule.

Example 32

medium

Derive the formula

V = \tfrac{4}{3}\pi r^3

for a sphere's volume using calculus, then explain in words.

Example 33

medium

Why does the chain rule

(f \circ g)' = f'(g) \cdot g'

multiply two derivatives? Explain conceptually.

Example 34

hard

The derivative of

\sin x

\cos x

. Explain why this makes geometric sense in terms of the unit circle.

Example 35

hard

Why does

\int_0^\infty e^{-x^2}\,dx = \tfrac{\sqrt\pi}{2}

involve

\pi

? Sketch the polar-coordinates explanation, not the full derivation.

Example 36

hard

Why does

\sum_{k=1}^{\infty} \frac{1}{k^2} = \frac{\pi^2}{6}

involve

\pi

? Outline (don't fully derive) the explanation via

\sin x

's roots.

Example 37

challenge

Two proofs that

\sqrt 2

is irrational: a parity-based contradiction and a unique-factorization argument. Which one offers a deeper EXPLANATION, and why?

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Related Concepts

Proof (Intuition)

Background Knowledge

These ideas may be useful before you work through the harder examples.

proof intuition