giúp mình với mng ơi

$R_{23}=$ $R_{2}+$ $R_{3}=40+20=60 Ω$

$R_{234}=$ $\frac{R_{23}.R_{4}}{R_{23}+R_{4}}=$ $\frac{60.40}{60+40}=24Ω$

$R=$ $R_{1}+$ $R_{234}=40+24=64Ω$

$I=$ $\frac{U}{R}=$ $\frac{64,8}{64}=$ $\frac{81}{80}A$

$U_{AC}=$ $U_{1}=I.$ $R_{1}=$ $\frac{81}{80}.40=40,5V$

$I_{2}=$ $\frac{R_{4}}{R_{23}+R_{4}}.I=$ $\frac{40}{60+40}.$ $\frac{81}{80}=0,405A$

$U_{AD}=$ $U_{1}+$ $U_{2}=40,5+$ $I_{2}$$R_{2}$

⇒$U_{AD}=40,5+0,405.40=57,6V$ 

Vì `R_2` nt `R_3` `=>` `R_(23)=R_2+R_3=40+40/2=60` `(\Omega)`

Vì `R_(23)` // `R_4` `=>` `R_(234)=(R_(23).R_4)/(R_(23)+R_4)=(60.40)/(60+40)=24` `(\Omega)`

Vì `R_(234)` nt `R_1` `=>` `R_(tđ)=R_(234)+R_1=24+40=64` `(\Omega)`

Ta có: `I=I_1=I_(234)=I_(U_(AB)/R_(tđ)=(64,8)/64=81/80=1,0125` `(A)`

`=>` `U_(AC)=U_1=I_1.R_1=1,0125.40=40,5` `(V)`

`=>` `U_(234)=U_(23)=U_(AB)-U_(AC)=64,8-40,5=24,3` `(V)`

`=>` `I_(23)=I_2=I_3=U_(23)/R_(23)=(24,3)/60=0,405` `(A)`

`=>` `U_2=I_2.R_2=0,405.40=16,2` `(V)`

`=>` `U_(AD)=U_(AC)+U_2=40,5+16,2=56,7` `(V)`

Vậy `U_(AC)=40,5V;U_(AD)=56,7V`

$#huy21072009$