outputTable = (From row In inputTable.AsEnumerable()
Group row By col1 = row.Field(Of String)("col1") Into Group
Select outputRow = inputTable.NewRow()
).CopyToDataTable()
For Each row In outputTable.Rows.Cast(Of DataRow)()
row("col2") = inputTable.AsEnumerable().
Where(Function(x) x.Field(Of String)("col1") = row.Field(Of String)("col1")).
Sum(Function(x) Convert.ToDouble(x.Field(Of String)("Col2")))
Next
(From d in DataTableOrigVar.AsEnumerable
Group d by k=d("Col1").ToString.Trim into grp=Group
Let sm = grp.Sum(Function (g) CDbl(g("Col2").toString.Trim)).Tostring("F2")
Let ra = new Object(){k,sm}
Select r =dtResult.Rows.Add(ra)).CopyToDataTAble