$X(0)=0$, $X'(1)=0$ picks $\sin\sqrt\lambda\,t$ with $\cos\sqrt\lambda=0$

Eigenvalues corresponding to the Sturm–Liouville problem $\dfrac{d^2X}{dt^2}+\lambda X=0$, $X(0)=0$, $X'(1)=0$ are $\lambda_n$, $n\in\mathbb{N}$, where $\lambda_n=$