These two questions are actually the first question of 20 13, which is a relatively simple type. The third question examines the expansion of functions into Fourier series and Dirichlet convergence theorem.

The fourth question examines the second kind of curve integral and Green's theorem (that is, it is converted into double integral to calculate). The fourth question, if you have a good understanding of double integral, you can draw four pictures at a glance. It is easy to judge whether the double integral in each picture is positive or negative. If you don't know how to ask me again, I just did the real question of 13 not long ago.