Maths Literacy Matric Revision: Exponential graphs

Revision Notes for CAPS Mathematical Literacy Grade 12: Patterns and Relationships – Exponential Graphs

Introduction

Exponential graphs are a crucial part of the Mathematical Literacy curriculum in Grade 12. Understanding these graphs helps us describe situations where quantities grow or decay at a constant percentage rate over time. You’ll learn to identify, sketch, and interpret exponential graphs, which have numerous applications in finance, science, and daily life.

Key Points

  1. Definition: An exponential graph represents situations where a quantity changes at a rate proportional to its current value, generally following the form ( y = a \cdot b^x ).

  2. Formula: The general formula for exponential growth or decay is:
    [
    y = a \cdot b^x
    ]
    where:

  3. ( y ) is the final amount
  4. ( a ) is the initial amount
  5. ( b ) is the growth (if ( b > 1 )) or decay (if ( 0 < b < 1 )) factor
  6. ( x ) is the exponent (often representing time)

  7. Properties:

  8. When ( b > 1 ): The graph shows exponential growth, curving upwards.
  9. When ( 0 < b < 1 ): The graph shows exponential decay, curving downwards.
  10. The y-intercept is always at ( (0, a) ).

  11. Graph Interpretation:

  12. Starting Point: At ( x = 0 ), ( y = a ).
  13. Growth Rate: Determined by the base ( b ).
  14. Horizontal Asymptote: The ( y )-axis approaches but never touches zero in decay graphs.

Real-World Applications

Example:
Population Growth: If a population of 100 bacteria grows by 20% every hour, the population ( P ) at time ( t ) hours is given by:
[
P = 100 \cdot (1.2)^t
]

Step-by-Step Solution:
1. Initial Population (( a )): 100
2. Growth Rate (( b )): 1.2
3. Formula: ( P = 100 \cdot (1.2)^t )
4. Calculate Population at 3 Hours:
[
P = 100 \cdot (1.2)^3 \approx 172.8
]

Common Misconceptions and Errors

  1. Confusing Growth and Decay Factors: Remember, ( b > 1 ) indicates growth, while ( 0 < b < 1 ) indicates decay.
  2. Incorrect Plotting: Ensure accurate plotting of points and correct interpretation of the graph’s curve.
  3. Misinterpreting Asymptotes: Recognize that exponential decay graphs approach the x-axis but never truly reach zero.

Practice and Review

Practice Questions:
1. A car depreciates in value at a rate of 15% per year. If the initial value is R150,000, what is its value after five years?

  1. A certain radioactive substance decreases by 10% every year. If the initial amount is 500 grams, how much remains after 3 years?

Solutions:
1. Use ( V = 150,000 \cdot (0.85)^5 ):
[
V \approx R85,737.59
]
2. Use ( A = 500 \cdot (0.9)^3 ):
[
A \approx 364.5 \text{ grams}
]

Examination Tips:
– Look for keywords like “percent increase per year” or “annual depreciation”.
– Use clear and labeled axes for graphing.
– Manage your time by solving simpler questions first.

Connections and Extensions

  • Finance: Understanding exponential growth (interest, investments) and decay (depreciation, loans).
  • Science: Radioactive decay, population growth, and other natural phenomena.

Summary and Quick Review

Key Points:
– Exponential functions follow ( y = a \cdot b^x ).
– Growth if ( b > 1 ), decay if ( 0 < b < 1 ).
– The y-intercept is ( (0, a) ).

Additional Resources


To dive deeper into exponential graphs and their applications in Mathematical Literacy, consult your textbook for more examples and exercises【4:1†source】【4:6†source】.

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