An unexpected SQL statement Problem

I found it when I wrote a statement to calculate the loan interest. Strsql = "Update plus_bank set savemoney = savemoney (1 + @ lixi)-@ qukuan, savedate = @ savedate where userid =" + userid;
This error occurs during executenonquery, And the savemoney function cannot be found. However, the following statement can be used. It is strange that the SQL statement has such limitations.
Strsql = "Update plus_bank set savemoney = savemoney + savemoney * @ lixi-@ qukuan, savedate = @ savedate where userid =" + userid;
Sqlcommand cmd3 = new sqlcommand (strsql, myconn );
Parameters 3.parameters. Add ("@ lixi", sqldbtype. Float, 4). value = Rate * (datetime. Now. Year-mysavedate.Year + (datetime. Now. Month-mysavedate.Month ));
Parameters 3.parameters. Add ("@ qukuan", sqldbtype. Int, 4). value = myqukuan;
Parameters 3.parameters. Add ("@ savedate", sqldbtype. datetime, 4). value = datetime. now;
Try
{
Dr. Close ();
Statement 3.executenonquery ();
}
Catch (exception ex)
{
Response. Write ("Q exception capture! "+ Ex );
Response. End ();
}