a) xét \(\Delta ABC\)và \(\Delta HBA\)có :
\(\widehat{BAC}=\widehat{BHA}=90^o\); \(\widehat{B}\)( chung )
\(\Rightarrow\Delta ABC\approx\Delta HBA\left(g.g\right)\)
b) \(\Delta ABC\approx\Delta HBA\left(g.g\right)\)\(\Rightarrow\frac{AB}{BC}=\frac{BH}{AB}\Rightarrow AB^2=BC.BH\)
c) xét \(\Delta BHA\)và \(\Delta CHA\)có :
\(\widehat{AHB}=\widehat{AHC}=90^o\); \(\widehat{ABH}=\widehat{A_1}\)( cùng phụ với \(\widehat{ACH}\))
\(\Rightarrow\Delta ABH\approx\Delta CAH\left(g.g\right)\)
\(\Rightarrow\frac{AB}{BH}=\frac{AC}{AH}\)
hay \(\frac{AB}{2BP}=\frac{AC}{2AQ}\)\(\Rightarrow\frac{AB}{BP}=\frac{AC}{AQ}\)
Xét \(\Delta ABP\)và \(\Delta CAQ\)có :
\(\frac{AB}{BP}=\frac{AC}{AQ}\); \(\widehat{ABP}=\widehat{A_1}\)
\(\Rightarrow\Delta ABP\approx\Delta CAQ\left(c.g.c\right)\)