Graphing the Cosine Function

Learn how to graph the cosine function, understand its key features, and explore transformations.

Advanced25 minLesson

Definition

The cosine function is written as and creates a smooth, wave-like curve called a sinusoid.
**Key features of :**
PropertyValue
Amplitude
Period (about )
Maximum at
Minimum at
Zeros
The cosine graph starts at its maximum when :
This is the key difference from sine, which starts at zero.

Try it now

What is the value of ?

Worked Examples

Graph one complete cycle of from to .

1

Find the starting point

Point:

2

Find the first zero

Point:

3

Find the minimum

Point:

4

Find the second zero

Point:

5

Find the endpoint (back to max)

Point:

Common Mistakes

Confusing sine and cosine starting points

Why it's wrong: Sine starts at (going up), while cosine starts at its maximum . Both are sinusoids but with different starting positions.

Correct: Remember: (starts at max), (starts at zero)

Getting phase shift direction wrong

Why it's wrong: The equation shifts RIGHT by , not left. The minus sign is counterintuitive.

Correct: shifts RIGHT; shifts LEFT

Forgetting to divide by for period

Why it's wrong: Students sometimes think has period , but period .

Correct: For , period , not

Interactive Visual

Unit Circle

Degrees

0°

Radians

0

sin(θ)

0

cos(θ)

1

tan(θ)

0

Coordinates (cos, sin)

(1, 0)

Click and drag to rotate the angle around the unit circle.

Linear Function Explorer

y = x
Slope (m)1
Y-Intercept (b)0
b
run
rise

Interactive Sandbox

Interactive Grapher

Try these examples:

y = 2x + 1

m=2, b=1

Expression Calculator

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History

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Practice Problems

16 problems
Problem 1 of 16
Easy

What is the value of ?

Why It Matters

The cosine function models countless real-world phenomena:
  • Sound waves: The pressure variations in sound follow cosine patterns
  • Alternating current (AC): Electrical current in your home oscillates like a cosine wave
  • Seasonal temperatures: Average temperatures throughout the year follow a cosine curve
  • Pendulum motion: The horizontal position of a swinging pendulum follows
  • Light waves: Electromagnetic radiation can be modeled using cosine functions
Understanding cosine graphs helps you analyze any periodic phenomenon!

Real World Applications

Modeling Daily Temperature

Daily temperature follows a cosine pattern, with maximum temperature in the afternoon and minimum at night.

Example:

If the average temperature is 20 degrees Celsius with a 10 degree variation, and maximum occurs at 3 PM, the model is: where is hours after midnight.

1Try It Yourself

A city has average temperature 25 degrees Celsius with variation of 8 degrees. Maximum is at 2 PM (14:00).

What is the temperature at midnight (t = 0)?

Step 1: Write the mathematical expression

Use and find :

Sound and Music

Musical notes are produced by sound waves that follow sinusoidal patterns. The cosine function models the pressure variation.

Example:

The note A4 (440 Hz) can be modeled as where is time in seconds and is amplitude.

2Try It Yourself

A sound wave is modeled by .

What is the frequency of this sound wave?

Step 1: Write the mathematical expression

Use the formula: frequency where :

Key Takeaways

  • 1The cosine function creates a wave that starts at its maximum when
  • 2Basic properties: amplitude , period , range
  • 3For : amplitude , period , phase shift , vertical shift
  • 4Cosine is a horizontal shift of sine:

Frequently Asked Questions

Cosine and sine have the same shape but different starting points. Cosine starts at its maximum (), while sine starts at zero (). Mathematically, .
Cosine and sine have the same shape but different starting points. Cosine starts at its maximum (), while sine starts at zero (). Mathematically, .
The period is because cosine is defined using the unit circle, and going around the circle once (360 degrees or radians) brings you back to the starting point.
A negative amplitude like reflects the graph across the -axis. The wave is flipped upside down: it starts at (minimum) instead of (maximum).

Glossary

Amplitude
The height from the midline to the maximum (or minimum). For , amplitude .
Period
The horizontal length of one complete cycle. For , period .
Phase shift
A horizontal translation of the graph. For , the shift is units to the right.
Vertical shift
Moving the entire graph up or down. For , the midline becomes .
Sinusoid
A wave-shaped curve like sine or cosine.

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