Just a few comments about why you might be getting some of the other errors in your worksheet:
- to use the RiemannSum command you need to load the package Student[Calculus1]
- when you assigned the variable c you just had a colon you need to use := to assign a variable
- to resolve the "use midpoint method instead" error you can use "method=bvp[middefer]" in your command for Q2
However, looking at your functions, the system seems to be undefined. The a2 expression has the fourth derivative of f (f(4)) and when solving the system Q2 using dsolve, Maple will compute f(4)= . Since you have the condition B1:=f(0)=0 this will be undefined. This is why you get the singularity error.
You may need to redefine your boundaries or adjust your expressions.