Problem Gradient Calculation false function?

Hey everybody, I ran into difficulties with the problem 101 Gradient Calculation. With a pocket calculator I get the same values as with the implemented function...but I always get wrong values in the final result.

I guess I wrote the function down wrong, because with the example function ( f(x,y) = xx + yy ) everything works fine. But I do not see where exactly the error is. Can someone give me a hint?