XQuery syntax to evaluate math formula

I want to evaluate a math formula inside of a function, so I can't use dynamic SQL.

I found that the query on an XML-variable with a literal works well.

 DECLARE @sParsedFormula varchar(200); 
 DECLARE @xFormula xml;
 SET @xFormula = '';
 SET @sParsedFormula = '1+2';
 SELECT @xFormula.query('3+3') , @xFormula.query('sql:variable("@sParsedFormula")')

Output (1st value is correctly evaluated, 2nd not):

"6", "1+2"

I can't directly pass @sParsedFormula to the query otherwise I get "The argument 1 of the XML data type method "query" must be a string literal".

What is the correct XQuery-syntax to evaluate the SQL:variable?

Tx,

Patrick