Learn on PengiAoPS: Introduction to Algebra (AMC 8 & 10)Chapter 16: Functions

Lesson 16.6: Operations

In this Grade 4 AoPS Introduction to Algebra lesson, students learn how to define and evaluate custom operations — such as x ★ y or a # b — by treating them as two-variable functions and applying substitution. Drawing from Chapter 16 on Functions, the lesson covers how standard operations like addition and multiplication are simply functions in familiar notation, then challenges students with AMC and MATHCOUNTS problems involving nested operations and solving equations with custom-defined operators.

Section 1

Custom Operation Definition

Property

A custom operation uses a symbol (like \star, #\#, &\&, \oplus) to represent a specific mathematical rule involving two variables. The general form is ab=expression involving a and ba \circ b = \text{expression involving } a \text{ and } b, where \circ is the custom symbol.

Examples

Section 2

Nested Custom Operations

Property

When custom operations are nested, evaluate from the innermost operation outward, following the order of operations. For nested operations like a(bc)a \star (b \star c), first evaluate (bc)(b \star c), then apply the operation aresulta \star \text{result}.

Examples

Section 3

Function Iteration vs Function Multiplication

Property

Function iteration notation: fn(x)f^n(x) means applying function ff exactly nn times, so f2(x)=f(f(x))f^2(x) = f(f(x))

Function multiplication: f(x)f(x)=[f(x)]2f(x) \cdot f(x) = [f(x)]^2 means multiplying the output by itself

Section 4

Function Composition

Property

The composition of functions f and g is written fgf \circ g and is defined by

(fg)(x)=f(g(x))(f \circ g)(x) = f(g(x))

We read f(g(x))f(g(x)) as ff of gg of xx. In composition, the output of one function is the input of a second function.

Examples

Book overview

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Chapter 16: Functions

  1. Lesson 1

    Lesson 16.1: The Machine

  2. Lesson 2

    Lesson 16.2: Combining Functions

  3. Lesson 3

    Lesson 16.3: Composition

  4. Lesson 4

    Lesson 16.4: Inverse Functions

  5. Lesson 5

    Lesson 16.5: Problem Solving with Functions

  6. Lesson 6Current

    Lesson 16.6: Operations

Lesson overview

Expand to review the lesson summary and core properties.

Expand

Section 1

Custom Operation Definition

Property

A custom operation uses a symbol (like \star, #\#, &\&, \oplus) to represent a specific mathematical rule involving two variables. The general form is ab=expression involving a and ba \circ b = \text{expression involving } a \text{ and } b, where \circ is the custom symbol.

Examples

Section 2

Nested Custom Operations

Property

When custom operations are nested, evaluate from the innermost operation outward, following the order of operations. For nested operations like a(bc)a \star (b \star c), first evaluate (bc)(b \star c), then apply the operation aresulta \star \text{result}.

Examples

Section 3

Function Iteration vs Function Multiplication

Property

Function iteration notation: fn(x)f^n(x) means applying function ff exactly nn times, so f2(x)=f(f(x))f^2(x) = f(f(x))

Function multiplication: f(x)f(x)=[f(x)]2f(x) \cdot f(x) = [f(x)]^2 means multiplying the output by itself

Section 4

Function Composition

Property

The composition of functions f and g is written fgf \circ g and is defined by

(fg)(x)=f(g(x))(f \circ g)(x) = f(g(x))

We read f(g(x))f(g(x)) as ff of gg of xx. In composition, the output of one function is the input of a second function.

Examples

Book overview

Jump across lessons in the current chapter without opening the full course modal.

Continue this chapter

Chapter 16: Functions

  1. Lesson 1

    Lesson 16.1: The Machine

  2. Lesson 2

    Lesson 16.2: Combining Functions

  3. Lesson 3

    Lesson 16.3: Composition

  4. Lesson 4

    Lesson 16.4: Inverse Functions

  5. Lesson 5

    Lesson 16.5: Problem Solving with Functions

  6. Lesson 6Current

    Lesson 16.6: Operations