Can I get inline an array element?
I mean something like this:
={LINEST(B8:B23;C8:C23;1;1)}[row_index, col_index]LibreOffice Calc only supports Inline constant arrays.
The syntax local to your install of LibO is configurable concerning the separators.
Using the same separators as used when saving a spreadsheet document to a .ods (.fods) file
{1;2;3|4;5;6} would describe a matrix with two rows of three elements per row like:
1 2 3
4 5 6
Elements, rows, or columns can be referred to by INDEX() the same way as with arrays from ranges or from results returned under array-evaluation.
Unfortunately locales having the point as decimal separator mess this up replacing the semicolon with the comma by default.
I would recommend to change this via >Tools>Options>LibreOffice Calc>Formula>>Separators.
Expressions (formulas) in place of array elements are still not supported.
Specifically your question now:
I suppose your fancy formula actually tried to get somethiing like:
=INDEX(LINEST(B8:B23;C8:C23;1;1); RowIndex; ColumnIndex)
If so the question is not actually related to inline arrays.
Please note: Working with LINEST() you will always need to evaluate more than one element of the returned array.
To access the elements by more than one formula of the mentioned kind is a grave mistake since every such formula needs to perform the complete evaluation for LINEST().
Use a dedicated range for the output of LINEST() without exception.