Đáp án:

\(\begin{array}{l}
a.\\
{t_{nt}} = 45p\\
b.\\
{t_{//}} = 10p
\end{array}\)

Giải thích các bước giải:

\(\begin{array}{l}
a.\\
{Q_1} = {Q_2}\\
 \Rightarrow {P_1}t = {P_2}t\\
 \Rightarrow \dfrac{{{U^2}}}{{{R_1}}}.900 = \dfrac{{{U^2}}}{{{R_2}}}.1800\\
 \Rightarrow {R_1} = 0,5{R_2}\\
{R_{nt}} = {R_1} + {R_2} = 0,5{R_2} + {R_2} = 1,5{R_2}\\
{Q_{nt}} = {P_{nt}}{t_{nt}} = \dfrac{{{U^2}}}{{{R_{nt}}}}{t_{nt}} = \dfrac{{{U^2}}}{{1,5{R_2}}}{t_{nt}}\\
{Q_{nt}} = {Q_2}\\
 \Rightarrow \dfrac{{{U^2}}}{{1,5{R_2}}}{t_{nt}} = \dfrac{{{U^2}}}{{{R_2}}}.1800\\
 \Rightarrow {t_{nt}} = 2700s = 45p\\
b.\\
{R_{//}} = \dfrac{{{R_1}{R_2}}}{{{R_1} + {R_2}}} = \dfrac{{0,5{R_2}{R_2}}}{{0,5{R_2} + {R_2}}} = \dfrac{{{R_2}}}{3}\\
{Q_{//}} = {P_{//}}{t_{//}} = \dfrac{{{U^2}}}{{{R_{//}}}}{t_{//}} = \dfrac{{{U^2}}}{{\dfrac{1}{3}{R_2}}}{t_{//}}\\
{Q_{//}} = {Q_2}\\
 \Rightarrow \dfrac{{{U^2}}}{{\dfrac{1}{3}{R_2}}}{t_{//}} = \dfrac{{{U^2}}}{{{R_2}}}.1800\\
 \Rightarrow {t_{//}} = 600s = 10p
\end{array}\)