Hello deltaxrayFirst how to avoid the use of ^ for superscripts.

- Take the expression you wrote: x^2 + y^2 = r^2

- Select it with the mouse (or <Shift>+<cursor keys>)

- Press the button on the toolbar - this will wrap it round with [tex] and [/tex] tags.

- Check the result using the 'Preview Post' button: .

The scripting language that does this is called LaTeX. There's lots of help on this forum for it. If you want to see any of the LaTeX code on what I've written below, just click it with the mouse.

Then:

The origin is the centre of the circle . So the gradient of the radius passing through is .

The tangent is perpendicular to this line; so its gradient is .

So the equation of the tangent is:

i.e.

or, since is a point on the circle .Grandad