a) \(\dfrac{S_{HBC}}{S_{ABC}}=\dfrac{\dfrac{1}{2}.HA'.BC}{\dfrac{1}{2}.AA'.BC}=\dfrac{HA'}{AA'}\)
Tương tự: \(\dfrac{S_{HAB}}{S_{ABC}}=\dfrac{HC'}{CC'};\dfrac{S_{HAC}}{S_{ABC}}=\dfrac{HB'}{BB'}\)
\(\dfrac{HA'}{AA'}+\dfrac{HB'}{BB'}+\dfrac{HC'}{CC'}=\dfrac{S_{HBC}}{S_{ABC}}+\dfrac{S_{HAC}}{S_{ABC}}+\dfrac{S_{HAB}}{S_{ABC}}=\dfrac{S_{HAB}+S_{HAC}+S_{HAB}}{S_{ABC}}=\dfrac{S_{ABC}}{S_{ABC}}=1\)
b) Áp dụng tính chất đường phân giác vào các tam giác: ADC; ABI; AIC, ta có:
\(\dfrac{BI}{IC}=\dfrac{AB}{AC};\dfrac{AN}{NB}=\dfrac{AI}{BI};\dfrac{CM}{MA}=\dfrac{IC}{AI}\)
\(\dfrac{BI}{IC}.\)\(\dfrac{AN}{AB}.\)\(\dfrac{CM}{MA}=\dfrac{AB}{AC}.\)\(\dfrac{AI}{BI}.\)\(\dfrac{IC}{AI}=\dfrac{AB}{AC}.\)\(\dfrac{IC}{BI}\)
\(\Rightarrow BI.AN.CM=BN.IC.AM\)