As you will see from the many links on this blog there are many high quality Mathematics resources available on the Internet. The collection here was inspired by a recent test taken by my Year 10 (ages 14-15) students. You can use these to explore many topics you study.
There are many excellent reference materials online, see the reference page on this blog.
You could look up expression, equation and formula in Jenny Eather’s dictionary for example to help you understand the difference between these terms. On the subject of expressions, equations and formulae, a related term you should be familiar with is identity. You could use this glossary for teachers to look it up.
The spreadsheet used here is of the extensive collection on Mike Hadden’s MathsFiles site (Trial and Error1dp).
On the subject of Excel, you could use it to plot points and draw a graph; you could then fit a trend line as shown in the example below. Examining the numbers here you should recognise the powers of 2, we have y = 1000×2x.
Mathisfun has many useful Geometry resources for example you could explore quadrilaterals with this interactive and this area calculation tool allows you to check the area of several common shapes.
To calculate the area of a triangle if you know two sides and the included angle you could use this WolframAlpha widget.
Try this widget for the cosine rule.
Communicating Mathematics online can be tricky! An online whiteboard can be the answer, I have used Scriblink here:
(For other online whiteboard resources see this post)