$\tiny{{s4}.{13}.{5}.{41}}$

$\textsf{find if planes are $\parallel, \perp$ or $\angle$ of intersection }\\$

\begin{align}

\displaystyle

{P_1}&={x+z=1}\\

\therefore n_1&=\langle 1,0,1 \rangle\\

\\

{P_2}&={y+z=1}\\

\therefore n_2&=\langle 0,1,1 \rangle\\

\\

\cos(\theta)&=

\frac{n_1\cdot n_2}{|n_1||n_2|}\\

&=\frac{1(0)+0(1)+1(1)}

{\sqrt{1+1}\cdot\sqrt{1+1}}

=\frac{1}{2}\\

\cos^{-1}\left({\frac{1}{2}}\right)&=

\color{red}{60^o}

\end{align}

$\textit{there are 2 more problems like this so presume this is best method.. }\\$

$\textit{didn't know if it is

common notation to call a plane $P_1$}$