Cũng đơn giản thôi !
Ta có :
\(\left\{{}\begin{matrix}P_1=\dfrac{f}{s}\\P_1=\dfrac{F}{S}\end{matrix}\right.\)
\(\Rightarrow\) \(\dfrac{f}{s}=\dfrac{F}{S}\rightarrow f.S=F.s\)
\(\rightarrow\) \(\dfrac{F}{f}=\dfrac{S}{s}\)
Ta có : \(P_1=\dfrac{f}{s}\) ; \(F_1=\dfrac{F}{S}\)
\(\Rightarrow\) \(\dfrac{f}{s}=\dfrac{F}{S}\) \(\rightarrow\) \(f.s=F.S\)
\(\Rightarrow\) \(\dfrac{F}{f}=\dfrac{S}{s}\)(đpcm)