a. Xet ΔBAD va ΔBED co BD chung; BA = BE; ^ABD = ^EBD (BD phan giac cua ^B)
=> ΔBAD = ΔBED (cgc) => AD = DE
b. Vi ΔBAD = ΔBED => ^BAD = ^BED = 90.
Xet ΔADK va ΔEDC co DA = DE; ^DAK = ^DEC = 90, ^ADK = ^EDC (doi dinh)
=> ΔADK = ΔEDC (gcg) => DK = DC => ΔKDC can tai D
c. Ap dung Pytago => AC^2 = NC^2 - AB^2 = 100 - 36 = 64 = 8^2=> AC = 8