Goi M là trung điểm BC, M' là trung điểm B'C'.
$\dfrac{dt(SA' G')}{dt(SAG)}=\dfrac{dt(SA'G)}{\dfrac{2}{3}dt(SAM)}=\dfrac{SA'.SG'}{SA.SG}$
$\dfrac{dt(SA' G')}{dt(SAM)}=\dfrac{2SA'.SG'}{3SA.SG}(1)$
$\dfrac{dt(SG' M')}{dt(SGM)}=\dfrac{dt(SG'M')}{\dfrac{1}{3}dt(SGM)}=\dfrac{SG'.SM'}{SG.SM}$
\Rightarrow$ \dfrac{dt(SG'M')}{dt(SAM)}=\dfrac{SG'.SM'}{3SG.SM}(2)$
Cộng 1 và 2:
$\dfrac{dt(SA'G')+dt(S' G' M')}{dt(SAM)}=\dfrac{dt(SA'M')}{dt(SAM)} (3)$
\Rightarrow $\dfrac{SG'}{3SG}(\frac{2SA'}{SA}+\dfrac{SM'}{SM})= \dfrac{SA'.SM'}{SA.SM}(4)$
$\dfrac{dt(SB'M')}{dt(SBC)}+\dfrac{dt(SM'C')}{dt(SBC)}=\dfrac{dt(SB'C')}{dt(SBC)}$
$\dfrac{1}{2}[\dfrac{dt(SB' M')}{dt(SBM)}+\dfrac{dt(SM'C')}{dt(SMC)}]=\dfrac{dt(SB'C')}{dt(SBC)}$
\Leftrightarrow $\dfrac{1}{2}(\dfrac{SB'.SM'}{SB.SM}+\dfrac{SM'.SC'}{SM.SC})$=$\dfrac{SB'.SC'}{SB.SC}(5)$
đặt $\dfrac{SA'}{SA}=a$, $\dfrac{SB'}{SB}=b$, $\dfrac{SC'}{SC}=c $
(5):$\dfrac{1}{2}\dfrac{SM'}{SM}(b+c)=bc$
$\dfrac{SM'}{SM}=\dfrac{2bc}{b+c}$
(4) \Rightarrow $\dfrac{SG'}{3SG}(2a+\dfrac{2bc}{b+c})=\dfrac{2abc}{b+c}$
$\dfrac{SG'}{3SG}=\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}$
\Leftrightarrow$\dfrac{SA}{SA'}+\dfrac{SB}{SB'}+ \dfrac{SC}{SC'}=\dfrac{3SG}{SG'}$
PS: em chưa học => chép sách:-\".