Math-o-mir is a free tool for writing and editing equations, a "personal math notepad where you can write down your quick and informal calculations".
A toolbar makes it easy to get started. Click the root symbol, the integral, whatever you need, then type in your values or variables.
Alternatively, there are lots of keyboard tricks. A single backslash allows you to enter commands (\sqrt gives you the square root symbol), a double-backslash enters the fraction sign, quick double keystrokes enter Greek symbols, and there are plenty more.
You're able to copy elements of one equation to another. It's not the regular mouse select/ copy/ paste system you might expect, but once you figure it out, works just fine.
There are also tools to plot or analyze functions, do basic freehand drawings and more, and the finished results may be exported as LaTeX code or regular images.