R1//R2=>Rtđ=\(\dfrac{R1.R2}{R1+R2}=\dfrac{U}{I}=\dfrac{220}{2}=110=\dfrac{R1.240}{R1+240}=>R1\sim203,1\Omega\)
Vậy..............
Tóm Tắt
R1 // R2
U = 220 V
I= 2A
R2=240\(\Omega\)
Tính R1=?
Giải
Ta có \(R_{tđ}=\dfrac{U_{AB}}{I_{AB}}=\dfrac{220}{2}=110\Omega\)
mà R1 // R2 => \(\dfrac{1}{R_{tđ}}=\dfrac{1}{R_1}+\dfrac{1}{R_2}\)
\(\Leftrightarrow\dfrac{1}{R_1}=\dfrac{1}{R_{tđ}}-\dfrac{1}{R_2}=\dfrac{1}{110}-\dfrac{1}{240}=\dfrac{13}{2640}\)
=>\(R_1=\dfrac{2640}{13}\Omega\)