C
Giải thích:
T=$\frac{20}{5}$ =0,4s
T= $\sqrt[2$\pi$]{$\frac{m}{k}$ }$
=$\sqrt[2$\pi$]{$\frac{Δl}{g}$ }$
⇒Δl=$\frac{T²g}{4$\pi$ ²}$ =$\frac{T²}{4}$ =$\frac{0,4²}{4}$ =0,04m=4cm
$\frac{$F_{dh max}$ }{$xF_{dh max}$ }$=$\frac{k(Δl+A)}{k(Δl-A}$
=$\frac{Δl+A}{Δl-A}$ =$\frac{4+3}{4-3}$ =7
CHÚC BN HOK TỐT!!!!