Đáp án:

\(\begin{array}{l}
a.{t_{//}} = 8p\\
b.{t_{nt}} = 50p
\end{array}\)

Giải thích các bước giải:

\(\begin{array}{l}
a.\\
{A_1} = {A_2}\\
 \Rightarrow \dfrac{{{U^2}}}{{{R_1}}}.{t_1} = \dfrac{{{U^2}}}{{{R_2}}}.{t_2}\\
 \Rightarrow \dfrac{{{t_1}}}{{{R_1}}} = \dfrac{{{t_2}}}{{{R_2}}}\\
 \Rightarrow \dfrac{{10}}{{{R_1}}} = \dfrac{{40}}{{{R_2}}}\\
 \Rightarrow {R_2} = 4{R_1}\\
{R_{//}} = \dfrac{{{R_1}.{R_2}}}{{{R_1} + {R_2}}} = \dfrac{{{R_1}.4{R_1}}}{{{R_1} + 4{R_1}}} = 0,8{R_1}\\
\frac{{{t_{//}}}}{{{R_{//}}}} = \frac{{{t_1}}}{{{R_1}}}\\
 \Rightarrow {t_{//}} = \dfrac{{{t_1}{R_{//}}}}{{{R_1}}} = \dfrac{{10.0,8{R_1}}}{{{R_1}}} = 8p\\
b.\\
{R_{nt}} = {R_1} + {R_2} = {R_1} + 4{R_1} = 5{R_1}\\
{t_{nt}} = \dfrac{{{t_1}{R_{nt}}}}{{{R_1}}} = \dfrac{{10.5{R_1}}}{{{R_1}}} = 50p
\end{array}\)