giúp em `18,19` với ạ

Câu 18:

\(\begin{array}{l}
{R_1} = 2{R_2} \Rightarrow {I_2} = 2{I_1}\\
{I_1} + {I_2} = 1,25 \Rightarrow \left\{ \begin{array}{l}
{I_1} = \dfrac{5}{{12}}A\\
{I_2} = \dfrac{5}{6}A
\end{array} \right.\\
{R_1} = \dfrac{U}{{{I_1}}} = \dfrac{{60}}{{\dfrac{5}{{12}}}} = 144\Omega \\
{R_2} = \dfrac{U}{{{I_2}}} = \dfrac{{60}}{{\dfrac{5}{6}}} = 72\Omega 
\end{array}\)

Câu 19:

a) Ta có:

\(\begin{array}{l}
{R_{nt}} = 6,25.{R_{ss}}\\
 \Rightarrow {R_1} + {R_2} = 6,25.\dfrac{{{R_1}{R_2}}}{{{R_1} + {R_2}}}\\
 \Rightarrow {\left( {{R_1} + {R_2}} \right)^2} = 6,25{R_1}{R_2}\\
 \Rightarrow R_1^2 - 4,25{R_1}{R_2} + R_2^2 = 0\\
 \Rightarrow {\left( {\dfrac{{{R_1}}}{{{R_2}}}} \right)^2} - 4,25.\dfrac{{{R_1}}}{{{R_2}}} + 1 = 0\\
 \Rightarrow \left[ \begin{array}{l}
\dfrac{{{R_1}}}{{{R_2}}} = 4\\
\dfrac{{{R_1}}}{{{R_2}}} = \dfrac{1}{4}
\end{array} \right.
\end{array}\)

b) 

Ta có:

\(\left\{ \begin{array}{l}
{R_1} + {R_2} = 100\\
{R_1} = 4{R_2}
\end{array} \right. \Rightarrow \left\{ \begin{array}{l}
{R_2} = 20\Omega \\
{R_1} = 80\Omega 
\end{array} \right.\)