$$(n+t^{2}x^{2})^{\frac{3}{2}}\ge n^{\frac{3}{2}}$$
$$\implies\frac{1}{(n+t^{2}x^{2})^{\frac{3}{2}}}\le\frac{1}{n^{\frac{3}{2}}}$$
$$\implies\int_{0}^{1}\frac{1}{(n+t^{2}x^{2})^{\frac{3}{2}}}dt\le\frac{1}{n^{\frac{3}{2}}}$$
$$\implies\sum_{n=1}^{\infty}\int_{0}^{1}\frac{1}{(n+t^{2}x^{2})^{\frac{3}{2}}}dt\le\sum_{n=1}^{\infty}\frac{1}{n^{\frac{3}{2}}}$$