= Solution
A <Givens rotation> is identity except for a $2\times2$ block $\begin{pmatrix}c&s\\-s&c\end{pmatrix}$ in rows $p,q$. Choose $p=j,q=i$ and $c=a_{jj}/r$, $s=a_{ij}/r$, $r=(a_{jj}^2+a_{ij}^2)^{1/2}$, with sign adjusted to the block convention; this mixes only those rows and zeros $a_{ij}$. For $3\times3$, successively apply rotations $(1,2)$, $(1,3)$, $(2,3)$, producing patterns $***;0**;***$, then $***;0**;0**$, then an upper triangular $R$. The product of transposed rotations is $Q$.
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