Exponentials and Logarithms

Q: Can I use natural logarithm (ln) to solve equations in P2?

No: natural logarithms and the function are part of the P3 syllabus, and exam markers will penalize unnecessary use of out-of-scope content even if your final answer is correct. Use base 10 logarithms for all P2 exponential equation questions.

Edexcel International A-Level Mathematics· 2018 Specification (Issue 3, first assessment 2019)· 25 min read

1. Graphs of Exponential Functions $y=a^x$★★☆☆☆⏱ 7 min

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📘 Definition

General base exponential function

A function of the form $y = a^{x}$ where $a > 0$ , $a \neq = 1$ , and $x$ is any real number.

Example:

$y = 2^{x}$ and $y = (1/2)^{x}$ are both valid exponential functions

All exponential graphs of the form $y = a^{x}$ share two fixed features: they pass through the point $(0, 1)$ (since any non-zero number raised to the power of 0 is 1) and have a horizontal asymptote at $y = 0$ (the x-axis, as the function never touches or crosses the x-axis). For $a > 1$ , the graph is strictly increasing, growing rapidly as $x$ becomes large positive, and approaching 0 as $x$ becomes large negative. For $0 < a < 1$ , the graph is strictly decreasing, approaching 0 as $x$ becomes large positive, and growing as $x$ becomes large negative.

📐 Worked Example

Sketch the graphs of $y = 4^{x}$ and $y = (1/4)^{x}$ on the same axes, labelling all key points and asymptotes.

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Step 1: Identify shared key features: both graphs pass through $(0, 1)$ and have a horizontal asymptote at $y = 0$ .
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Step 2: For $y = 4^{x}$ : $a = 4 > 1$ , so it is an increasing function. Plot an additional point at $x = 1$ : $y = 4$ , so add the coordinate $(1, 4)$ to your sketch.
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Step 3: For $y = (1/4)^{x}$ : $a = 1/4 < 1$ , so it is a decreasing function. Plot an additional point at $x = 1$ : $y = 1/4$ , so add the coordinate $(1, 0.25)$ to your sketch.
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Step 4: Label the axes, the asymptote $y = 0$ , the intercept $(0, 1)$ , your additional plotted points, and clearly label each graph.

Exam tip:

Always label the y-intercept at $(0, 1)$ and the horizontal asymptote $y = 0$ on exponential graph sketches; these are required for full marks even if not explicitly requested.

2. Laws of Logarithms★★★☆☆⏱ 10 min

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📘 Definition

Logarithm base a

$log_{a} x$

For $a > 0$ , $a \neq = 1$ and $x > 0$ , $log_{a} x = y$ if and only if $a^{y} = x$ . Logarithms are the inverse operation of exponentiation.

Example:

$log_{2} 8 = 3$ because $2^{3} = 8$

The four core logarithm laws below must be memorized, as they are not provided in the exam formula booklet. All laws require that $a > 0$ , $a \neq = 1$ , $x > 0$ , and $y > 0$ to be valid:

Product law: $log_{a} (x y) = log_{a} x + log_{a} y$
Quotient law: $log_{a} (\frac{x}{y}) = log_{a} x - log_{a} y$
Power law: $log_{a} (x^{k}) = k log_{a} x$ for any real constant $k$
Reciprocal law: $log_{a} (\frac{1}{x}) = - log_{a} x$ (derived from the power law with $k = - 1$ )
Key identity: $log_{a} a = 1$ (since $a^{1} = a$ )

📐 Worked Example

Simplify $3 log_{10} 2 + log_{10} 25 - log_{10} 2$ into a single logarithm, then evaluate its numerical value.

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Step 1: Apply the power law to the first term: $3 log_{10} 2 = log_{10} (2^{3}) = log_{10} 8$
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Step 2: Combine the first two terms using the product law: $log_{10} 8 + log_{10} 25 = log_{10} (8 \times 25) = log_{10} 200$
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Step 3: Subtract the third term using the quotient law: $log_{10} 200 - log_{10} 2 = log_{10} (\frac{200}{2}) = log_{10} 100$
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Step 4: Evaluate using the key identity: $log_{10} 100 = log_{10} (1 0^{2}) = 2 log_{10} 10 = 2 \times 1 = 2$

Exam tip:

Show every step of log manipulation in your working; missing a law application step will cost you method marks even if your final answer is correct.

3. Solving Exponential Equations of the Form $a^x = b$★★★★☆⏱ 8 min

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To solve equations where the unknown variable is in the exponent, take logarithms of both sides of the equation, then use the power law to bring the exponent down as a multiplier. You can use any valid base for your logarithms, but base 10 is recommended for P2 as it is available on all standard calculators. The change of base formula, provided in your exam formula booklet, confirms that $log_{a} b = \frac{log _{c} b}{log _{c} a}$ for any positive $c \neq = 1$ .

📐 Worked Example

Solve the equation $3^{2 x + 2} = 75$ , giving your answer to 3 significant figures.

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Step 1: Take base 10 logarithms of both sides of the equation: $log_{10} (3^{2 x + 2}) = log_{10} 75$
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Step 2: Apply the power law to the left-hand side: $(2 x + 2) log_{10} 3 = log_{10} 75$
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Step 3: Rearrange to isolate the bracket term: $2 x + 2 = \frac{log _{10} 75}{log _{10} 3}$
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Step 4: Calculate the right-hand side using your calculator, keeping full precision: $\frac{log _{10} 75}{log _{10} 3} \approx 3.9299$
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Step 5: Rearrange to solve for x: $2 x = 3.9299 - 2 = 1.9299$ , so $x \approx 0.965$ (3 significant figures)
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Step 6: Verify your answer by substituting back into the original equation: $3^{2 (0.965) + 2} = 3^{3.93} \approx 75$ , which matches the right-hand side.

Exam tip:

Never round intermediate values when solving exponential equations, as this introduces rounding errors that can make your final answer fall outside the allowed tolerance range. Only round your final answer to the requested number of significant figures.

4. Common Pitfalls

Wrong move:

Using natural logarithm (ln) or the $e^{x}$ function to solve P2 exponential equations

Why:

Natural exponential and log functions are part of the P3 syllabus, and markers will penalize use of out-of-scope content even if your final answer is correct.

Correct move:

Use base 10 logarithms for all P2 exponential equation questions, as they are in scope and available on all standard calculators.

Wrong move:

Applying log laws to negative or zero argument values

Why:

Logarithms are only defined for positive arguments, so manipulating $log (- 5)$ or $log (0)$ is mathematically invalid, leading to lost marks.

Correct move:

Always check that all arguments in your log expressions are positive before applying any log laws.

Wrong move:

Forgetting the identity $log_{a} a = 1$ when simplifying log expressions

Why:

This identity is not listed in the formula booklet, so many students overlook it and overcomplicate simplification questions.

Correct move:

Memorize $log_{a} a = 1$ and use it to simplify expressions where the base and argument of a log match.

Wrong move:

Omitting the horizontal asymptote $y = 0$ when sketching exponential graphs

Why:

Exponential graphs never touch the x-axis, so the asymptote is a required feature for full marks in graph questions.

Correct move:

Always label the $y = 0$ asymptote and the $(0, 1)$ y-intercept when drawing any $y = a^{x}$ graph.

Wrong move:

Rounding intermediate calculation values when solving exponential equations

Why:

Early rounding introduces cumulative errors that often lead to final answers outside the exam mark scheme's tolerance range.

Correct move:

Keep all intermediate values stored in your calculator at full precision, only round your final answer to the required number of significant figures.

5. Quick Reference Cheatsheet

Concept	Formula/Rule	Key Notes
Exponential graph $y = a^{x}$	Passes through $(0, 1)$ , asymptote $y = 0$	Increasing if $a > 1$ , decreasing if $0 < a < 1$
Product log law	$log_{a} (x y) = log_{a} x + log_{a} y$	Must memorize; $a, x, y > 0, a \neq = 1$
Quotient log law	$log_{a} (\frac{x}{y}) = log_{a} x - log_{a} y$	Must memorize; $a, x, y > 0, a \neq = 1$
Power log law	$log_{a} (x^{k}) = k log_{a} x$	Must memorize; $a, x > 0, a \neq = 1$
Reciprocal log law	$log_{a} (\frac{1}{x}) = - log_{a} x$	Derived from power law, $k = - 1$
Solve $a^{x} = b$	$x = \frac{log _{c} b}{log _{c} a}$	Change of base formula provided in booklet
Key log identity	$log_{a} a = 1$	Must memorize for simplification

6. Frequently Asked

Do I need to memorize the log laws for P2 exams?

Yes: the product, quotient, power, and reciprocal log laws are not included in the formula booklet, so you must memorize them. Only the change of base formula is provided.

Can I use natural logarithm (ln) to solve $a^{x} = b$ equations in P2?

No: natural logarithms and the function $e^{x}$ are part of the P3 syllabus, and exam markers will penalize unnecessary use of out-of-scope content even if your final answer is correct. Use base 10 logarithms for all P2 exponential equation questions.

Going deeper

What's Next

Now that you have mastered P2 exponentials and logarithms, you are ready to progress to more advanced related content in Pure Mathematics 3 (P3), including the natural exponential function $e^{x}$ and natural logarithm $ln x$ , as well as log-linear graphing techniques to estimate parameters for models of the form $y = a x^{n}$ and $y = k b^{x}$ . This topic also forms a critical foundation for working with exponential growth and decay models in Statistics 1, and differential equations in later pure and applied units. Make sure you complete enough past paper practice for this topic to build speed and accuracy, as log manipulation is a required skill for almost all higher-level Edexcel IAL Maths units.