Racquel Sanderson

Find $\overrightarrow{AB}$ and $\overrightarrow{AC}$
$\overrightarrow{AB}=\begin{pmatrix}-2\\0\\2\end{pmatrix}-\begin{pmatrix}1\\2\\3\end{pmatrix}=\begin{pmatrix}-3\\-2\\-1\end{pmatrix}$
$\overrightarrow{AC}=\begin{pmatrix}2\\-4\\-1\end{pmatrix}-\begin{pmatrix}1\\2\\3\end{pmatrix}=\begin{pmatrix}1\\-6\\-4\end{pmatrix}$

Find the cross product $\overrightarrow{AB}\times \overrightarrow{AC}$
$\begin{pmatrix}-3\\-2\\-1\end{pmatrix}\times\begin{pmatrix}1\\-6\\-4\end{pmatrix}=\begin{pmatrix}2\\-13\\20\end{pmatrix}$
This is the normal, $overrightarrow{n}$ , to the plane.
We know $A$ is on the plane

$\begin{equation*}\overrightarrow{n}\cdot (\overrightarrow{r}-\overrightarrow{OA})=0\end{equation}$

Hence $\overrightarrow{n}\cdot \overrightarrow{r}=\overrightarrow{n}\cdot \overrightarrow{OA}$
$\overrightarrow{n}\cdot \overrightarrow{r}=\begin{pmatrix}2\\-13\\20\end{pmatrix} \cdot \begin{pmatrix}1\\2\\3\end{pmatrix}=38$
Therefore the Cartesian equation of the plane is

$\begin{equation*}2x-13y+20z=38\end{equation}$

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Filed under Algebra, Cross Product, Vectors, Year 12 Specialist Mathematics

April 6, 2026 · 9:23 am

The Secret Life of Numbers – Kate Kitagawa and Timothy Revell

The first place I went to buy this was sold out! Seems somewhat amazing – perhaps they only had one copy. I found it at my favourite book store – Subiaco Bookshop.

Here’s the blurb …

Mathematics shapes almost everything we do. But despite its reputation as the study of fundamental truths, the stories we have been told about it are wrong. In The Secret Lives of Numbers, historian Kate Kitagawa and journalist Timothy Revell introduce readers to the mathematical boundary-smashers who have been erased by history because of their race, gender or nationality.

From the brilliant Arabic scholars of the ninth-century House of Wisdom, and the pioneering African American mathematicians of the twentieth century, to the ”lady computers” around the world who revolutionised our knowledge of the night sky, we meet these fascinating trailblazers and see how they contributed to our global knowledge today.

This revisionist, completely accessible and radically inclusive history of mathematics is as entertaining as it is important.

This has a lovely style and is very easy to read. As part of my maths degree, I studied some history, but it was very western and I enjoyed the global approach in this book. I do think it is accessible and anyone with an interest in maths or history could read it.

A review

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Filed under Book Review

Tagged as book review, kate kitagawa, the secret life of numbers, timothy revell

March 21, 2026 · 7:47 am

Complex Loci

Sketch on an Argand diagram

$\begin{equation*}\lvert{z-6}\rvert-\lvert{z+6}\rvert=3\end{equation}$

where $z\in \mathbb{C}$

Let $z=x+yi$

$\begin{equation*}\lvert{x+yi-6}\rvert-\lvert{x+yi+6\rvert=3\end{equation}$

$\begin{equation*}\lvert{x-6+yi}\rvert-\lvert{x+6+yi\rvert=3\end{equation}$

$\begin{equation*}\sqrt{(x-6)^2+y^2}-\sqrt{(x+6)^2+y^2}=3\end{equation}$

$\begin{equation*}\sqrt{(x-6)^2+y^2}=3+\sqrt{(x+6)^2+y^2}\end{equation}$

Square both sides of the equation

$\begin{equation*}{(x-6)^2+y^2=9+6\sqrt{(x+6)^2+y^2}+(x+6)^2+y^2\end{equation}$

$\begin{equation*}x^2-12x+36+y^2-9-x^2-12x-36-y^2=6\sqrt{(x+6)^2+y^2}\end{equation}$

(1) $\begin{equation*}-24x-9=6\sqrt{(x+6)^2+y^2}\end{equation*}$

From equation $1$ we know $-24x-9\ge0$

Hence $x\le\frac{-3}{8}$ , which means we only have the left section of the hyperbola.

$\begin{equation*}-8x-3=2\sqrt{(x+6)^2+y^2}\end{equation}$

Square both sides of the equation

$\begin{equation*}(-8x-3)^2=4((x+6)^2+y^2)\end{equation}$

$\begin{equation*}64x^2+48x+9=4x^2+48x+144+y^2\end{equation}$

$\begin{equation*}60x^2-y^2=135\end{equation}$

$\begin{equation*}\frac{4x^2}{9}-\frac{y^2}{135}=1\end{equation}$

(2) $\begin{equation*}\frac{x^2}{\frac{9}{4}}-\frac{y^2}{135}=1\end{equation*}$

Remember, we have the left part of the hyperbola.

The $x-$ intercept $=-\sqrt{\frac{9}{4}}=-\frac{3}{2}$ and the asymptotes are $y=\pm \frac{\sqrt{135}}{\frac{3}{2}}x$

$y=\pm 2\sqrt{15}x$

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Filed under Algebra, Complex Numbers, Simplifying fractions, Sketching Complex Regions, Solving Equations, Year 12 Specialist Mathematics

Tagged as complex loci, hyperbola

March 11, 2026 · 5:34 am

Function Composition

If $g(f(x))=\frac{x}{x+1}$ and $f(x)=\frac{1}{1-2x}$ , find $g(x)$ .

$\begin{equation*}g(f(x))=g(\frac{1}{1-2x})=\frac{x}{x+1}\end{equation}$

Sometimes you can do this type of question by inspection, but this one is a bit harder. I am going to use a variable substitution.

Let $u=\frac{1}{1-2x}$

$\begin{equation*}u=\frac{1}{1-2x}\end{equation}$

$\begin{equation*}1-2x=\frac{1}{u}\end{equation}$

$\begin{equation*}1-\frac{1}{u}=2x\end{equation}$

$\begin{equation*}\frac{u-1}{u}=2x\end{equation}$

$\begin{equation*}x=\frac{u-1}{2u}\end{equation}$

Therefore

$\begin{equation*}g(u)=\frac{\frac{u-1}{2u}}{\frac{u-1}{2u}+1}\end{equation}$

$\begin{equation*}g(u)=\frac{\frac{u-1}{2u}}{\frac{u-1+2u}{2u}}\end{equation}$

$\begin{equation*}g(u)=\frac{u-1}{3u-1}\end{equation}$

Therefore

$\begin{equation*}g(x)=\frac{x-1}{3x-1}\end{equation}$

Let’s test it

$g(x)=\frac{x-1}{3x-1}$ and $f(x)=\frac{1}{1-2x}$

$\begin{equation*}g(f(x))=\frac{\frac{1}{1-2x}-1}{\frac{3}{1-2x}-1}\end{equation}$

$\begin{equation*}g(f(x))=\frac{\frac{1-(1-2x)}{1-2x}}{\frac{3-(1-2x)}{1-2x}}\end{equation}$

$\begin{equation*}g(f(x))=\frac{2x}{2+2x}\end{equation}$

$\begin{equation*}g(f(x))=\frac{x}{x+1}\end{equation}$

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Filed under Algebra, Composition, Functions, Simplifying fractions, Year 12 Specialist Mathematics

Tagged as function composition

February 25, 2026 · 1:22 pm

Complex Locus Question

My Year 12 Specialist students are working on complex loci again. The following type of question always creates confusion.

$arg(z-w_1)$ is the angle the vector from $w_1$ to $z$ makes with the positive $x-$ axis, likewise for $arg(z-w_2)$ .

I am going to plot a possible $z$ and try to see the geometry that works.

We want $arg(z-w_1)-arg(z-w_2)=\frac{\pi}{6}$

I am going to take advantage of some triangle geometry

Using the External Angle Theorem, we know $\alpha=\beta+\theta$

$\begin{equation*}arg(z-w_1)-arg(z-w_2)=(\alpha+\frac{\pi}{2})-(\beta+\frac{\pi}{2}0=\alpha-\beta=\theta\end{equation}$

Therefore $\theta=\frac{\pi}{6}$

So we want all of the $z$ values that have an angle of $\frac{\pi}{6}$

Now we are going to use some circle geometry -The angle at the circumference subtended by the same arc are congruent. So we need to find a circle that has those three points ( $z, w_1$ and $w_2$ ) on the circumference.

Hence the locus is

Now we need to find the radius and centre of the circle.

Using another circle theorem, the angle at the centre is twice the angle at the circumference.

The triangle must be equilateral (it is isosceles with a vertex angle of $\frac{\pi}{3}$ )

Hence the radius is 2.

$h=\sqrt{2^2-1^2}=\sqrt{3}$

Hence the centre is $(-\sqrt{3}+1, 0)$

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Filed under Circle Theorems, Complex Numbers, Geometry, Interesting Mathematics, Pythagoras, Sketching Complex Regions, Year 12 Specialist Mathematics

Tagged as complex loci

February 11, 2026 · 3:28 am

Complex Numbers and Trig Idenities

My Year 12 Specialist Students are using complex numbers to prove trigonometric identities.

Things like

$\begin{equation*}sin(5\theta)=16sin^5(\theta) -20sin^3(\theta)+5sin(\theta) \end{equation}$

Method 2 might be a little bit easier depending upon how your brain works.

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Filed under Algebra, Binomial Expansion Theorem, Complex Numbers, Identities, Trig Identities, Trigonometry, Year 12 Specialist Mathematics

Tagged as complex numbers, de moivre's theorem, trigonometric identities

February 1, 2026 · 8:42 am

Drawing Threefold Symmetry with Geogebra

I am an embroiderer and I have been experimenting with geometric drawings. Like the diagram below.

I have been using Geogebra – I use the desktop app.

(1) Start with a circle (mine has a radius of 5)

(2) Place 6 points equidistant around the circle (I placed a point on the circle and then used the angle of a given size tool – 60 degrees)

I have hidden the angle markers to make everything a bit easier to see.

(3) Draw circles (with the same radius as the original) on every second point.

(4) Find the distance between two centre points of the new circles

(5) Using the centres of the three new circles, draw circles where the radius is the distance you found in (4).

(6) Mark the points of intersection.

(7) Hide the big circles

(8) Draw arcs

Finished – change the colour (if you want) and remove labels, etc.

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Filed under Drawing, Geometry, Interesting Mathematics

Tagged as geometry, threefold symmetry

January 22, 2026 · 9:27 am

Complex Loci Question

A sketch of the locus of a complex number $z$ is shown above, determine the maximum value of $arg(z)$ correct to two decimal places where $0\le z \le 2\pi$