We have the 3 following gates :
$$ H = \dfrac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\ 1 & -1 \end{bmatrix} $$ $$ R(\varphi) = \begin{bmatrix}1 & 0 \\ 0 & e^{-i\varphi} \end{bmatrix} $$ $$ R(\psi) = \begin{bmatrix}1 & 0 \\ 0 & e^{i\psi} \end{bmatrix} $$
and we want to construct a one-bit circuit that produces the final state $$ |\Xi\rangle = \cos {\varphi\over{2}} |0\rangle + e^{i\psi}\sin {\varphi\over{2}} |1\rangle $$
I do not understand how a factor $\cos {\varphi\over{2}} $ can appear in front of $|0\rangle$, can someone help me ?