# Exponentials and Logarithms

A-Level Mathematics Year 1: Exponentials and Logarithms

Throughout the unit, students progress from understanding how exponents can be written using logarithms to transforming exponential models into straight-line graphs.

Lessons on Integration

4 Part Lesson
4 Part Lesson
4 Part Lesson
4 Part Lesson
4 Part Lesson
4 Part Lesson
4 Part Lesson
4 Part Lesson
##### Transforming Exponential Models

Prerequisite Knowledge

From GCSE

• Calculate with roots, and with integer and fractional indices
• Simplifying expressions involving sums, products and powers, including the laws of indices
• Calculate exactly with surds
• Simplify surd expressions involving squares and rationalise denominators

From AS-Level Mathematics

• Solve quadratic equations (including those that require rearrangement) algebraically by:
• factorising,
• completing the square using the quadratic formula;
• Find approximate solutions using a graph.

Success Criteria

• Know and use the function ax and its graph, where a is positive
• Know and use the function ex and its graph.
• Know that the gradient of ekx is equal to kekx and hence understand why the exponential model is suitable in many applications.
• Know and use the definition of Loga x as the inverse of ax, where a is positive and x ≥ 0.
• Know and use the function Ln x and its graph
• Know and use Ln x as the inverse function of ex.
• Understand and use the laws of logarithms
\log _{a} x+\log _{a} y=\log _{a}(x y)
\log _{a} x-\log _{a} y=\log _{a}\left(\frac{x}{y}\right)
k \log _{a} x=\log _{a} x^{k}
• Solve equations of the form ax = b.
• Use logarithmic graphs to estimate parameters in relationships of the form y = axn and y = kbx, given data for x and y.
• Understand and use exponential growth and decay; use in modelling (examples may include the use of e in continuous compound interest, radioactive decay, drug concentration decay, exponential growth as a model for population growth); consideration of limitations and refinements of exponential models]

Teaching Points

• Logarithms answer the question: ‘To what power do I need to raise to get ?’
• You can only take a logarithm of a positive number.
• Logarithms in base 10 are written as log x.
• e is approximately 2.718…
• 10x and Log10x are opposite functions.
• ex and Ln x are also opposite functions.
• The laws of logarithms and special cases

If y = ax then x = Loga y

\log _{a} x+\log _{a} y=\log _{a}(x y)
\log _{a} x-\log _{a} y=\log _{a}\left(\frac{x}{y}\right)
k \log _{a} x=\log _{a} x^{k}
\log _{a}\left(\frac{1}{x}\right)=-\log _{a} x
e^{\ln x}=\operatorname{Ln}\left(e^{x}\right)=x
• Equations of the form a2x can be treated as a quadratic equation.
• If y = kbx then
\log y=x \log b+\log k

The graph of Log y against x has gradient Log b and intercept Log k.

Misconceptions

• When solving simple equations of the form 3x = 8 students often show correct working but lose accuracy marks with their final answer.
• Many students struggle to transform an exponential model into the form y = mx + c.
• Some students confuse the exponential graph with that of a parabola or cubic failing to appreciate that y = ax has y-intercept of 1.
• When setting up quadratics from equations of the type e2x – 7ex + 12 = 0 students attempt to solve using the quadratic formula when factorisation is more efficient.
• Some students misuse the laws of logarithms, for instance when attempting to use the multiplication and division laws they incorrectly write Log (x + 15) = Log x + Log 15 or Log (x – 2) as Log(x/2).

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