
theorem Thm16:
  for x being Real holds (sin (-x))^2=(sin x)^2
  proof
    let x be Real;
    (sin(-x))^2 = sin(-x) * sin(-x) by SQUARE_1:def 1
    .= (- sin x) * sin (-x) by SIN_COS:31
    .= (- sin x) * (- sin x) by SIN_COS:31
    .= (sin x) *  (sin x);
    hence thesis by SQUARE_1:def 1;
  end;
