create table dbo.test (ID int IDENTITY( 1 , 1 ), Formula varchar( 100 ))
go
insert into dbo.test (Formula) values('1 + 1')
insert into dbo.test (Formula) values('power(2, 2) - 2')
insert into dbo.test (Formula) values('sqrt(9) - 1')
insert into dbo.test (Formula) values('log10(10) + 1')
go
create function dbo.Calc(@Formula varchar( 100 ))
returns real AS
BEGIN
  declare @Result real
  set @Formula = 'select ' + @Formula
  select @Result = Res from openquery(SERVER, 'exec @Formula')  -- Must declare the variable @Formula
 
  select @Result = Res from openquery(SERVER, @Formula)  --Incorrect syntax near @Formula
 
  exec sp_executesql @Formula, N'@Result real out', @Result = @Result out  -- Only functions and extended stored procedures can be executed from within a function.
 
  return @Result
END
GO
select ID, dbo.Calc(Formula) from dbo.test
go
drop function dbo.Calc
go
drop table dbo.test
go