... AB.BC = 82 8 23 BC = 82 8 BC = 82 8 : 23 = 36 cm 1 SBEC = BC.CE = 36 .8 = 144 m2 2 SABEC = SABCD + SBEC = 82 8 + 144 = 9 72 m2 Hng dn v nh: ễn li ni dung bi hc Lm Bt : 27 -31 / sgk/ T 125 + 126 Tun ... di BC bng 2cỏch C1: theo chng minh trờn ta cú: DE 2 = BC = DE = 25 (cm) BC 5 C2: Da vo kớch thc ó cho ta cú: 6 -8- 10 ADE vuụng A BC2 = AB2 + AC2 = 1 52 + 20 2 = 625 BC = 25 D 28 , 5 C Hs trỡnh ... 12, 4 52+ 20 ,5 02 =23 , 98 b) cho AB = 12, 5cm; AC = 20 ,50cm Tớnh AB AC BC BC, AH, BH, CH = = T cú: HB HA BA HB = AB 12, 4 52 = = 6, 46 BC 23 , 98 cm AC AB 12, 45 .20 ,50 = = 10,64 cm HC = BC BC 23 , 98 HB...