a. K1, K2 mở => R2 nt R1
\(\Rightarrow Rtd=R1+R2=....\)
b,b. K1 mở, K2 đóng=>(R2 nt R1)//R5
\(\Rightarrow Rtd=\dfrac{R5.\left(R1+R2\right)}{R5+R1+R2}=....\)
c,c. K1 đóng, K2 mở=> R2 nt {R1//(R3 nt R4)}
\(\Rightarrow RTd=R2+\dfrac{R1.\left(R3+R4\right)}{R1+R3+R4}=....\)
d, K1, K2 đóng=>R5 //{R2 nt {R1 //(R3ntR4)}}
\(\Rightarrow RTd=\dfrac{R5.\left\{R2+\dfrac{R1.\left(R3+R4\right)}{R1+R3+R4}\right\}}{R5+R2+\dfrac{R1.\left(R3+R4\right)}{R1+R3+R4}}=....\)