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

theorem Th7:
  sin(-PI/2) = -1 & sin.-PI/2 = -1
proof
  sin.(-PI/2) = sin.(-PI/2+2*PI) by SIN_COS:78
    .= -1 by SIN_COS:76;
  hence thesis by SIN_COS:def 17;
end;
