Question

Giúp em câu 1b ạ

Answer 1

B1 :

Câu b :

<=> \(4.10^{-5}=\dfrac{4.2.10^{-7}.20}{r}\Rightarrow\dfrac{4.2.10^{-7}.20}{4.10^{-5}}=0,4m=40cm\)