Quite by accident, I discovered a really nice free software product from Microsoft called Microsoft Mathematics. You can download the standalone version from here. There is also a Microsoft Mathematics add-in for Microsoft Word and OneNote with the 2010 version found here and the 2013 version found here.
Figure 1. Standalone version of Microsoft Math showing the Formula and Equations dropdown menu selections.
Figure 2. Gravitational formula with Plot this equation menu option selected.
Figure 3. Graph clearly showing that the force of gravity declines with increasing distance.
The add-in for Word and OneNote adds the Mathematics tab to the ribbon, which builds upon existing features for entering equations. The add-in provides computational and graphing features.
Figure 4. Mathematics tab selected with the mouse over the Equation menu.
After selecting an equation, the DESIGN tab is added to the ribbon.
Figure 5. Microsoft Mathematics DESIGN tab in Word.
Figure 6. 3D graph showing how the area is a function of the square of the radius.
The add-in provides an option to insert a graph into Word or OneNote.
Figure 7. Graph embedded into Word document.
I found Microsoft Mathematics to be easy to use.