Issue in query() method of xquery in sql server

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SQL

Hi All,
I got a issue in query() method of xquery when i am trying to solve the arithmetic expression .
the problem is that
when i use the sql statments like this

DECLARE @x xml=''
DECLARE @x1 xml;
DECLARE @Result DECIMAL(38,8)
SET @x1= @x.query('1*12')
SET @Result= CAST(@x1.value('(/.)[1]', 'Float') AS DECIMAL(38,8))
Select @Result

then it returns the value 12.00000000 which is correct

but when i am going to use any variable in place of arithmetic expression like this


DECLARE @x xml=''
DECLARE @x1 xml;
DECLARE @Result DECIMAL(38,8)
DECLARE @r varchar(60)='1*12'
SET @x1= @x.query(@r)
SET @Result= CAST(@x1.value('(/.)[1]', 'Float') AS DECIMAL(38,8))
Select @Result

then it gives the error
The argument 1 of the XML data type method "query" must be a string literal.
i also tried
SET @x1= @x.query('sql:variable("@r")')

but it only gives only 1*12 not the value.

Please help me .