Using Eq. (6.2e), find the probability DDD of a particle of mass mmm and energy EEE tunnelling through the potential barrier shown in Fig. 6.6,6.6,6.6, where U(x)=U0(1−x2/l2)U(x)=U_{0}\left(1-x^{2} / l^{2}\right)U(x)=U0(1−x2/l2).