F''(x)=cosx, let f''(x)=0, then x=π/2+kπ, because it is a multiple-choice question, so take a symmetry center X = π/2 at will and calculate it.

F(π/2-x)+f(π/2+x)=2π, and it is observed that there is a data in the topic that is 5π, so a3=π/2 is just right. Substitute it into the calculation result and choose D.

Of course, this question is strictly proved to be a bit troublesome, so I won't fight. ...