I : Cho tam giác ABC nhọn , đường cao AH BK CN
a) C/m: \(\Delta ABK\sim\Delta ACN\)\
b) C/m: \(\left(\frac{AK}{AB}\right)^2=\frac{AN.NK}{AC.AB}\)
c) C/m: \(\frac{Sakn}{Sabc}=cos^2a\)
d) C/m: \(\frac{Shkn}{Sabc}=1-\left(cos^2a+cos^2b+cos^2c\right)\)
help me !!!