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

theorem Th10:
  cos.r = cos.(r+2*PI*i)
proof
  thus cos.r = cos r by SIN_COS:def 19
    .= cos(r+2*PI*i) by COMPLEX2:9
    .= cos.(r+2*PI*i) by SIN_COS:def 19;
end;
