Have you ever tried using the famous Equation Editor in Word 2003 and earlier? Then you probably know why many people prefer pen and paper or LaTeX rather than typing in equations in their Word documents.
In Word 2007 Microsoft has made your job as an equation-writer much easier. Go to the Insert tab, click Equation and then Insert New Equation (Alt+n+e+i). You are now in a new and improved version of the old equation editor. You can use the ribbon to enter equations much faster than you could in the old Equation Editor – and it is much easier, due to the ribbons great grouping capabilities.
But wait! There is an even easier way of doing it! The method explained above works great if you only have to enter a single equation or two, but if you have to enter lots of equations the mouse will kill you! So, when you want to write an equation, press Alt+= and now you are in math-mode – much faster than Alt+n+e+i 
Now you are ready to type in your equation (yes sir, type it in – look mom, no mouse 
). Let’s take an example.
In the Insert/Equation menu there are several built in equations you can use. We will enter the Binomial Theorem by hand. The first part of it says 
. So we start the math mode with Alt+= and now we simply type (x+a)^n where ^ means superscript. When we press =, Word will convert our text to a real equation – cool! We now have to enter the second part of the equation:
To get the summation symbol stuff we write \sum_(k=0)^n and press space to make Word convert our text into an equation. \sum is a Math AutoCorrect rule that converts to a summation symbol and _(k=0) makes k=0 a subscript of the summation symbol and as before ^n makes n a superscript. Easy? Yes sir!
For the last part we write (n\atop and press space so Word can convert \atop and then k)x^k and press space again and finally a^(n-k) and space. Now our equation should look like the built in formula:
A pretty nasty equation to construct using the mouse – but pretty simple in Word 2007
Unfortunately Microsoft did not put all the \functionname-stuff in the screentips (a new word for the advanced tooltips that is used in Office 2007). So it’s not that easy to learn how to write the equations. But if you have the link to Murray Sargent’s Unicode Nearly Plain-Text Encoding of Mathematics, you will have a very good reference
This feature is fantastic! So good luck with your next equation