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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)
