helpppppppppppppp 

hơi khó ạ , mong hsg giúp ạ 

Câu $\rm a)$ 

Ta có $\rm \dfrac{1}{R}=\dfrac{1}{R_1}+\dfrac{1}{R_2}$ 

`<=>`$\rm \dfrac{1}{R}=\dfrac{R_2+R_1}{R_1.R_2}$ 

`<=>`$\rm R=\dfrac{1}{\dfrac{R_2+R_1}{R_1.R_2}}$ 

`<=>`$\rm R=\dfrac{R_1.R_2}{R_1+R_2}$ 

Câu $\rm b)$ 

$\rm R=\dfrac{R_1.R_2}{R_1+R_2}=\dfrac{15.25}{15+25}=9,375(\Omega)$

`a)`

`1/R=1/[R_1]+1/[R_2]`

`=> 1/R=[R_1+R_2]/[R_1R_2]`

`=> R=[R_1R_2]/[R_1+R_2]`

`b)`

`R=[15*25]/[15+25]=375/40=9.375Omega`

`#2`