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If I did the integrations correctly, my answer is
[tex]
\langle n \vert \nabla_{a} \vert n \rangle = \int_{0}^{a}{dx \, \psi^{\ast}_{n}(x) \, \left( \frac{\partial}{\partial a} \psi_{n}(x) \right)} = -\frac{5}{2 a}
[/tex]
[tex]
\langle n \vert \nabla_{a} \vert n \rangle = \int_{0}^{a}{dx \, \psi^{\ast}_{n}(x) \, \left( \frac{\partial}{\partial a} \psi_{n}(x) \right)} = -\frac{5}{2 a}
[/tex]