Edexcel C3 — Question 3 8 marks

Exam BoardEdexcel
ModuleC3 (Core Mathematics 3)
Marks8
PaperDownload PDF ↗
Mark schemeDownload PDF ↗
TopicExponential Equations & Modelling
TypeNatural logarithm equation solving
DifficultyModerate -0.8 Part (a) is a straightforward one-step natural logarithm equation requiring only the definition of ln (exponentiating both sides) - a routine C3 skill. Part (b) requires finding a counter-example by testing values where the expression inside ln is less than 1, which is basic but requires slightly more thought than pure recall. Overall, this is easier than average A-level questions due to minimal steps and standard techniques.
Spec1.01c Disproof by counter example1.06g Equations with exponentials: solve a^x = b

3. (a) Solve the equation $$\ln ( 3 x + 1 ) = 2$$ giving your answer in terms of e.
(b) Prove, by counter-example, that the statement $$\text { "ln } \left( 3 x ^ { 2 } + 5 x + 3 \right) \geq 0 \text { for all real values of } x \text { " }$$ is false.

(a)
AnswerMarks
\(3x + 1 = e^2\)M1
\(x = \frac{1}{3}(e^2 - 1)\)M1 A1
(b)
AnswerMarks Guidance
consider \(\ln(3x^2 + 5x + 3) \geq 0\)M1
\(\Rightarrow 3x^2 + 5x + 3 \geq 1\)
\(3x^2 + 5x + 2 \geq 0\)
\((3x + 2)(x + 1) \geq 0\)M1
\(x \leq -1\) or \(x \geq -\frac{2}{3}\)A1
\(\therefore\) if (e.g.) \(x = -\frac{3}{4}, \ln(3x^2 + 5x + 3) = \ln\frac{15}{16} = -0.0645...\)M1
\(\therefore\) if \(x = -\frac{3}{4}, \ln(3x^2 + 5x + 3) < 0 \therefore\) statement is falseA1 (8)
**(a)**
$3x + 1 = e^2$ | M1 |
$x = \frac{1}{3}(e^2 - 1)$ | M1 A1 |

**(b)**
consider $\ln(3x^2 + 5x + 3) \geq 0$ | M1 |
$\Rightarrow 3x^2 + 5x + 3 \geq 1$ |
$3x^2 + 5x + 2 \geq 0$ |
$(3x + 2)(x + 1) \geq 0$ | M1 |
$x \leq -1$ or $x \geq -\frac{2}{3}$ | A1 |
$\therefore$ if (e.g.) $x = -\frac{3}{4}, \ln(3x^2 + 5x + 3) = \ln\frac{15}{16} = -0.0645...$ | M1 |
$\therefore$ if $x = -\frac{3}{4}, \ln(3x^2 + 5x + 3) < 0 \therefore$ statement is false | A1 | (8)
3. (a) Solve the equation

$$\ln ( 3 x + 1 ) = 2$$

giving your answer in terms of e.\\
(b) Prove, by counter-example, that the statement

$$\text { "ln } \left( 3 x ^ { 2 } + 5 x + 3 \right) \geq 0 \text { for all real values of } x \text { " }$$

is false.\\

\hfill \mbox{\textit{Edexcel C3  Q3 [8]}}