Quite an unassuming tip today, but potentially useful (and not immediately obvious).
In Excel, if you have a complex formula within a cell, you can immediately evaulate part of it by selecting the part (dragging across it with the mouse or by using arrow keys), then press F9.
For example, if you have:
then you select the 3+2/5 part:
and press F9:
This is useful for simplifying formulae, and also diagnosis when something isn't working as expected. Note that if you press enter (or click on another cell), it will save the formula with the evaluated number in the cell, rather than the original one. To avoid saving it, press Escape when you're done editing.
So for the above example, enter saves the formula =cos(3.4) in the cell, while pressing escape returns it to =cos(3+2/5).