still have problems,
1.
restart:
f := proc (x) options operator, arrow; piecewise(0 <= x and x < 4, 50, 4 <= x and x < 5, 0, 5 <= x, f(x-5)) end proc;
plot(proc (t) options operator, arrow; evalf(Int(f, 0 .. t, epsilon = 0.1e-3, digits = 15)) end proc, 0 .. 20);
in the above figure,there is a discont point in the interval [16..18],but in fact,it's a continuous curve.
2.
how to use fsolve deal with two periodic functions?