Đáp án: `S_2=0,16 \ mm^2`
Giải:
Ta có:
`R=\rho\frac{l}{S}`
Vì `\rho_1=\rho_2` nên
$\dfrac{R_1}{R_2}=\dfrac{\dfrac{l_1}{S_1}}{\dfrac{l_2}{S_2}}$
→ `\frac{R_1}{R_2}=\frac{l_1S_2}{l_2S_1}`
→ `\frac{60}{30}=\frac{150S_2}{30.0,4}`
→ `2=12,5S_2`
→ `S_2=0,16 \ (mm^2)`