reserve r,r1,r2, s,x for Real,
  i for Integer;

theorem Th8:
  sin.r = sin.(r+2*PI*i)
proof
  thus sin.r = sin r by SIN_COS:def 17
    .= sin(r+2*PI*i) by COMPLEX2:8
    .= sin.(r+2*PI*i) by SIN_COS:def 17;
end;
