Calca is a powerful symbolic calculator that updates as you type giving you instant answers. It solves equations and simplifies complex expressions. It's also a rich Markdown text editor so that you can explain your ideas alongside your calculations. This looks like a really interesting application,

Calculation Features:

• Variables x = 42
• Functions f(c) = 9/5*c + 32 and even recursive functions
• Units 42 mph in kmph and currency ¥599 in $
• Solves equations (x + 2x + 4x = 42, what is x?)
• Matrices with inverses so you can solve linear equations
• Sum, prod, map, and reduce operations on lists, matrices, and ranges
• Trigonometry and complex numbers
• Derivatives of functions
• Logic operations and if statements
• Binary math (& and |) and support for binary and hex number (0xA9 and 0b1010)
• Keeps track of units (m, s, m/s, m/s^2, etc.) so you can verify your equations as you compute

There is a comprehensive reference page that gives more details.

There is a MacOSX version and an iOS version for your iPad or iPhone.

The nice thing is you can use Markdown to format the calculation and add an explanation.

There is a listing of Markdown Editors for Mac OSX here,