Update app.py

app.py

@@ -175,7 +175,10 @@ NOTE: For eigenvalue problems, use 'lam = Symbol('lam')' instead of importing fr
    - For piecewise or discontinuous functions, compute limits at all points of interest, including boundaries.
 
    - **Important Note**: Always test limits symbolically first. If SymPy produces unexpected results, simplify the expression or expand it (e.g., using `series`) before re-evaluating the limit.
-   
+
+   8. If calculating integrals, write the SymPy code to compute the original integral directly with respect to the original variable of integration,
+      not any integral that might have been rewritten with a variable substitution.
+      
 ```python
 from sympy import Matrix, symbols, solve