pinkhappy

Cho $\fn_cm \triangle ABC$ , các góc đều nhọn, vẽ các đường cao AH, BK,CL. CM rằng:

a)($\fn_cm \frac{AK}{AB}$)$\fn_cm ^{2}$=$\fn_cm \frac{AL.AK}{AC.BC}$

b)$\fn_cm \frac{S_{AKL}}{S_{ABC}}$=cos$\fn_cm ^{2}$A

c)$\fn_cm \frac{S_{HKL}}{S_{ABC}}$= 1 - (cos$\fn_cm ^{2}$A + cos$\fn_cm ^{2}$B + cos$\fn_cm ^{2}$C)