H1
Rtđ = R1 +R2 =24
H2
1/Rtđ = 1/[tex]R_{23}[/tex]+1/[tex]R_{1}[/tex]
= 1/(R2 + R3) + 1/R1
=> Rtđ = 8
H3 R23 = R2+R3 =24
[tex]\frac{1}{R_{123}}[/tex] = [tex]\frac{1}{R_{23}}[/tex] + [tex]\frac{1}{R_{1}}[/tex]
=> [tex]\frac{1}{R_{123}}[/tex] = 8
Rtđ = [tex]R_{123}[/tex] + R = 20