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

theorem Th9:
  cos(-PI/2) = 0 & cos.-PI/2 = 0
proof
  cos.(-PI/2) = cos.(-PI/2+2*PI) by SIN_COS:78
    .= 0 by SIN_COS:76;
  hence thesis by SIN_COS:def 19;
end;
