5,
$\dfrac{1}{AB^2}+\dfrac{1}{AC^2}=\dfrac{1}{AH^2}$
\Leftrightarrow $\dfrac{1}{AH^2}=\dfrac{1}{27}+\dfrac{1}{81}=...$ \Rightarrow $AH=?$
7,$AH^2=BH.CH=9.16$ \Rightarrow $AH=12$
8,$AB^2=BH.BC=9(9+16)$ \Rightarrow $AB=15$
9,$MN=3a;MP=4a$ \Rightarrow $NP=5a$
Có: $3a.4a=5a.12$ \Rightarrow $a=0,2$
\Rightarrow $MP=4.0,2=...$