How to shade this triangle like this ?

[kingston123]

This is the diagram:

I attached this document where diagram is located. Please edit the diagram.

[Moonshine]

An option is to insert a Triangular Shape and use the Edit Shape > Edit Points option to manipulate the shape to a size/shape/position that you want.
The colour of the Shape (Shape Fill), border weight/colour can be also changed to your requirements via the Format tab.
It can then be r/clicked and put behind everything else so nothing is hidden using the Send to Back > Send to Back option.
Should you want to Group everything together to move/resize all as one, it’s often easier to view a list of multiple objects using the Selection Pane via the Home tab > Select > Selection Pane. Then minimise the initial Group.
Now select that Group and the additional showing entry (Right Triangle) in this example, then in the ‘document’, r/click one of the selected borders and opt to Group > Group.
Click the image below to see a short video of how I have done the above in Word 2010. Rather than someone doing the ‘editing’ for you, give it a try yourself.
I have minimised the Ribbon to see more of the document content.