Answer
A $\boldsymbol{B}_1$ paper is $707.107$ mm wide.
Work Step by Step
We have
$$
\begin{aligned}
g(1) & =\sqrt{f(1) \cdot f(1-1)} \\
& =\sqrt{f(1) \cdot f(0)}\\
&=\sqrt{594.605(840.896)}\\
& =707.107
\end{aligned}
$$ where $$\begin{aligned} f(1) & =1000 \cdot 2^{-\frac{1}{4}-\frac{1}{2}} \\ & =1000 \cdot 2^{-3 / 4} \\ & =594.604\end{aligned}$$ $$\begin{aligned} f(0) & =1000 \cdot 2^{-\frac{1}{4}-\frac{0}{2}} \\ & =1000 \cdot 2^{-1 / 4} \\ & =840.896\end{aligned}$$ This tells us that $\boldsymbol{B}_1$ paper is $707.107$ mm wide.
