I am used to work on microsoft word. there it is easy to insert science equations. Please help me how to insert equations or formula while writing notes.
The most basic answer to your question is 


A more detailed answer: If you use this feature a lot (I'm a math teacher, I use it a lot!) you can add a button to the toolbar to take you directly to the formula editor. There is a GUI input method, but you can also use a sortof command line version where you enter the code for the expression as text in your document, then highlight it and select
will turn into this Here's the a screenshot: (Notice the first symbol on the tool bar  that's where I've place the 


Use the Dmaths addon, which enables you to type the equation directly into the text e.g. : You would like to type an integral from a to b of f(x) dx, you just type " a;b;f(x) " ( without quotes ) and you type the shortcut "ctrl+shift+i" and there it is. I'm a math teacher and this is how I type all the equations I need. There's another addon that acts the same way called Cmath, but the first one is the most complete because you can also draw simple 3d figures, do plane geometry, and different other stuff. 

