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Bài 78:

Ta có:

\(\begin{array}{l}
{R_{12}} = {R_1} + {R_2} = 5 + 5 = 10\Omega \\
{R_{345}} = {R_3} + {R_4} + {R_5} = 5 + 5 + 5 = 15\Omega \\
{R_{AB}} = \dfrac{{{R_{12}}.{R_{345}}}}{{{R_{12}} + {R_{345}}}} = \dfrac{{10.15}}{{10 + 15}} = 6\Omega \\
I = \dfrac{U}{{{R_{AB}} + {R_6}}} = \dfrac{6}{{6 + 6}} = 0,5A\\
{U_4} = I.{R_{AB}} = 0,5.6 = 3V
\end{array}\)

Bài 79: 

Ta có:

\(\begin{array}{l}
{R_{234}} = {R_2} + {R_3} + {R_4} = 3 + 5 + 4 = 12\Omega \\
{R_{CD}} = \dfrac{{{R_{234}}.{R_5}}}{{{R_{234}} + {R_5}}} = \dfrac{{12.6}}{{12 + 6}} = 4\Omega \\
{R_{ACDB}} = {R_1} + {R_{CD}} + {R_6} = 8 + 4 + 12 = 24\Omega \\
R = \dfrac{{{R_7}.{R_{ACDB}}}}{{{R_7} + {R_{ACDB}}}} = \dfrac{{24.24}}{{24 + 24}} = 12\Omega \\
U = I.R = 1.12 = 12V\\
{I_{ACDB}} = \dfrac{U}{{{R_{ACDB}}}} = \dfrac{{12}}{{24}} = 0,5A\\
{I_3} = \dfrac{{{R_5}}}{{{R_{234}} + {R_5}}}.{I_{ACDB}} = \dfrac{6}{{12 + 6}}.0,5 = \dfrac{1}{6}A\\
{U_3} = {I_3}{R_3} = \dfrac{1}{6}.5 = \dfrac{5}{6}V
\end{array}\)