Ta có : SABC = $\frac{1}{2}$ . AB . AC . sinA
SAB'C' = $\frac{1}{2}$ . AB' . AC' . sinA
=> $\frac{SABC}{SAB'C'}$ = $\frac{\frac{1}{2} . AB . AC . sinA}{\frac{1}{2} . AB' . AC' . sinA}$
=> $\frac{SABC}{SAB'C'}$ = $\frac{AB . AC}{AB' . AC'}$ (đpcm).