Khi R1nt R2 thì :
Rtđ1=R1+R2
\(\Rightarrow\frac{U}{I}=R_1+R_2\)
\(\Rightarrow\frac{9}{0,5}=R_1+R_2\)
\(\Rightarrow R_1+R_2=18\Omega\)
Khi R1//R2 thì :
Rtđ2=\(\frac{R_1.R_2}{R_1+R_2}\)
\(\Rightarrow\frac{U}{I^,}=\frac{R_1.R_2}{R_1+R_2}\)
\(\Rightarrow\frac{9}{2,25}=\frac{R_1.R_2}{18}\)
\(\Rightarrow R_1R_2=72\)
\(\Rightarrow R_1=\frac{72}{R_2}\)\(\Omega\)
ta lại có : R1+R2=18
\(\Rightarrow\frac{72}{R_2}+R_2=18\)
\(\Rightarrow\frac{72+R^2_2}{R_2}=18\)
\(\Rightarrow72+R_2^2=18R_2\)
\(\Rightarrow72+R_2^2-18R_2=0\)
\(\Rightarrow R^2_2-12R_2-6R_2+72=0\)
\(\Rightarrow R_2\left(R_2-12\right)-6\left(R_2-12\right)=0\)
\(\Rightarrow\left(R_2-12\right)\left(R_2-6\right)=0\)
\(\Rightarrow\left[{}\begin{matrix}R_2=12\Omega\\R_2=6\Omega\end{matrix}\right.\)
Khi đó :
-Khi R2=12\(\Omega\Rightarrow R_1=18-12=6\Omega\)
-Khi R2=6\(\Omega\Rightarrow R_1=18-6=12\Omega\)
Vậy : \(\left\{{}\begin{matrix}R_1=6\Omega\\R_2=12\Omega\end{matrix}\right.;\left\{{}\begin{matrix}R_1=12\Omega\\R_2=6\Omega\end{matrix}\right.\)