`text{Bài 5}`

`R_{tđ} = R_1 + R_2 + R_3 = R_1 +R_1/2 + R_1/3 = {11}/6 R_1`

`text{Áp dụng định luật Ohm, ta được }`

`V = I . R_{tđ} => 60 = 3 . {11}/6 R_1 => R_1 = {120}/{11} \Omega`

`-> R_2 =R_1/2 =  {{120}/{11}}/2 = {60}/{11} \Omega`

`-> R_3 =R_1/3 =  {{120}/{11}}/3 = {40}/{11} \Omega`

`=> V_1 = I.R_1 = {360}/{11} V`

`=> V_2 = I.R_2 = {180}/{11} V`

`=> V_3 = I.R_3 = {120}/{11} V`

`text{Bài 6}`

`a)`

`1/R_{tđ} = 1/R_1 + 1/R_2 = 1/{50} + 1/{70} = {175}/6 \Omega`

`b)`

`I_1 = U/R_1  = {40}/{50} = 0,8 A`

`I_2 = U/R_2  = {40}/{70} = 4/7 A`

`=> I = I_1 + I_2 = 0,8 + 4/7 = {48}/{35} A`

`text{Bài 7}`

`a)`

`A = Pt = 1000 . (45.60) = 2700000J = 2700KJ`

`b)`

`R = U^2/P = 220^2/{1000} = 48,4 \Omega`

`P' = U'^2/R = 200^2/{48,4} ~~ 826,45W`

`=> A' = P' . t' = 826,45 . (30.60) ~~ 1487610J ~~ 1487,61kJ`

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