What is the difference between the instructions given in S1 and S2? Isn't it the same procedure?
On the first two problems, I want you to use the limit definition of the definite integral that uses Riemann sums. On the second two problems, I want you to quickly determine the value of the integral by just interpreting them as net signed area. That is, don't use the definition as in the first two problems, but rather stare at the picture and use some area formulas or determine that the net signed area is 0 because positive and negative stuff cancels each other out.