when I copy a math equation with some text from libreoffice wiriter and paste it to impress the text only copied but equation not copied? why??
The Writer viev wrongly encourages the idea text an shapes could be selected (and then copied e.g.) at the same time.
In fact an ordinary selection only contains text ranges, and a selection of shapes only shapes. If you copy text ranges containing shapes anchored to them, however, a Copy/Paste sequence performed inside Writer will do as you expect, because the shapes are treated as text content then. Draw and Impress use a much less powerful text model. Texts pasted into drawing documents always need to be moved into shapes there. Shapes cannot contain other shapes or anything anchored to their texts, and therefore also not OLE objects .
You need to copy/paste your formulas extra. The pasted formulas are inserted as OLE then into the slide/page directly.
===Edit 1h later===
Missed to mention that a formula for which it is assured that no further editing will occur might better be inserted as a GDI metafile (Paste Spezial..., CDtrl+Shift+V).