
theorem Th1:
  for r,s,t,u being Real st s <> 0 & t <> 0 &
    r^2 = s^2 + t^2 - 2*s*t*u holds u = (r^2 - s^2 - t^2)/(-2*s*t)
  proof
    let r,s,t,u be Real;
    assume that
A1: s <> 0 and
A2: t <> 0 and
A3: r^2 = s^2+t^2-2*s*t*u;
    r^2-s^2-t^2 = u * (-2*s*t) by A3;
    hence thesis by A1,A2,XCMPLX_1:89;
  end;
