how we get integration sign?

Lets see.

In the insert drop down box is special characters     

the integra sign is 3rd row 9th across        ∫     the rest I would just enter by normal typing.

OR

if i enter      int(3x)      int the maths formula input box     I get      $$\int{\left({\mathtt{3}}{\mathtt{\,\times\,}}{\mathtt{x}}\right)}{d{\mathtt{x}}}$$

 

OR

if you use the   LaTex Formula input box     you can type in   \int(3x)dx  and you will get    $$\int(3x)dx$$