# GraphicMaths

Visualising maths

## Cartesian equation of ellipse

The Cartesian equation of an ellipse is: $$\frac{x^2}{a^2} + \frac{y^2}{b^2} = 1$$ Understanding the equation Starting from the parametric equations: $$x = a \cos{t}\\ y = b \sin{t}$$ These can be rearranged to give: $$\cos{t} = \frac{x}{a}\\ \sin{t} = \frac{y}{b}$$ We also know that: $$\cos^2{t} + \sin^2{t} = 1$$ This follows from Pythagoras. Substituting for $\cos{t}$ and $\sin{t}$ gives Read more →

## Ellipses

An ellipse is a stretched circle. The longest diameter, $PQ$, is called the major axis. The shortest diameter, $RS$, is called the minor axis. The major and minor axes are always perpendicular. An ellipse can be defined by the parameters $a$ and $b$, where the major axis has length $2a$ and the minor axis has length $2b$ (or vice versa). An ellipse centred on the origin with its major and minor axes aligned with the x and y axes looks like this: Read more →

## Parametric equation of ellipse

The Parametric equation of an ellipse is: $$x = a \cos{t}\\ y = b \sin{t}$$ Understanding the equations We know that the equations for a point on the unit circle is: $$x = \cos{t}\\ y = \sin{t}$$ Multiplying the $x$ formula by $a$ stretches the shape in the x direction, so that is the required width (crossing the x axis at $x=\pm a$: Read more →

## Parabolas - effect of parameter ‘a’

This section looks at the effect of changing the parameter $a$ in the Cartesian equation of the parabola $y^2 = 4 a x$. Changing the value of $a$ moves the position of the focus and the directrix, which in turn changes the curve. The smaller the value of $a$, the closer the focus and directrix are to the origin. Use the buttons to set the value of $a$ to different values, and see the effect on the curve. Read more →

## Parabolas

We have previously seen how a parabola is defined in terms of parametric equations or alternatively in Cartesian form. An alternative way to define a parabola is as a locus of points. Focus and directrix The locus defining a parabola depends on a focus and a directrix. The focus is a point. For a standard parabola, the focus is located on the x axis a distance $a$ from the origin, that is at the point (a, 0). Read more →

## Lissajous figures

A simple Lissajous figure can be created using the parametric equations: \begin{align} x = sin(a t)\\ y = sin(b t) \end{align} This animated graph shows how $x$ and $y$ vary with $t$ to create the curve. We are using $a = 3$ and $b = 2$: The curve is cyclical. As $t$ varies from 0 to 2π radians, the curve traces a complete cycle and returns to the start point (0, 0). Read more →

## Other parametric curves

There are many other interesting parametric curves, in addition to parabolas, hyperbolas and ellipses. Some are show in this section. Read more →

## Cartesian equation of a rectangular hyperbola

We can convert the parametric equation of a hyperbola into a Cartesian equation (one involving only $x$ and $y$ but not $t$). Here are the parametric equations: \begin{align} x = c t\\ y = \frac{c}{t} \end{align} We can eliminate $t$ from these equations simply by multiplying $x$ and $y$: \begin{align} x y &= c t \times \frac{c}{t}\\ x y &= \frac{c^2 t}{t}\\ x y &= c^2 \end{align} Read more →

## Rectangular hyperbola

A rectangular hyperbola has the parametric equations: \begin{align} x = c t\\ y = \frac{c}{t} \end{align} Where $c$ is a positive constant, and $t$ is the independent variable. We can plot this curve by calculating the values of $x$ and $y$ for various values of $t$, and drawing a smooth curve through them. Curve for c = 1 Assuming $a = 1$, the parametric equations simplify to: Read more →