第一章综合练习第19题答案
∵ DE∥AB
∴∠BAC = ∠E,∠B=∠EDC
∴ △ABC ∽ △EDC
∴ AB:DE=AC:CE
∵ AD是∠BAC的外角平分线,DE∥AB
∴ ∠EDA=∠EAD
∴ DE = AE = AC + CE
∴ AB:(AC + CE)= AC:CE
即15:(12 + CE)= 12:CE
∴ CE = 48 m
第一章综合练习第20题答案
(1)作PD₁ ⊥ BC,垂足为D₁;作PD2∥AC,交BC于D2;作PD3∥BC交AC于D3
(2)4条(略)
第一章综合练习第21题答案
(1)不位似,
∵ NQ/QC = 2
MN/PQ = AN/AQ = 3/5
∴ 两梯的边不成比例
(2)∵ AN∶NQ:QC = 3:2:1
S△AMN:S△ABC =(AN/AC)2 = 1/4
∴ S△AMN = 1/4S△ABC,
同理,S△APQ = 25/36S△ABC,
∴ S梯形MNQP = S△APQ - S△AMN = 403(cm2)
第一章综合练习第22题答案
(1)略;
(2)3对;
(3)设正方形边长为x,则b - xb = xa,x = aba + b,
∴ S正方形CDEF/S△ABC = 2ab(a + b)2
第一章综合练习第23题答案
(1)PM = PN,
证明:∵ AP是等腰Rt△ABC斜边上的中线,
∴ ∠PAB = ∠C = 45°,PC = PA,
∵ ∠APC = 90°,
∴ ∠CPN = ∠APM,
∴ △CPN ≌ △APM(ASA),
∴ CN = AM,PN = PM,
(2)∵ PN = PM,∠EPF = 90°,
∴ ∠PMD = 45° = ∠C,
∵ ∠CPN = ∠DPM,
∴ △PCN ∽ △PMD,
DM/NC = PM/PC,
DM/AM = DM/NC = 4/5
∴ PM/PC = 4/5,PN/PC = 4/5
∵ PC = 1/2BC = 2
∴ PN = 4/5
过P作PH ⊥ AC,垂足为H,则△CHP为等腰直角三角形,
∵ P为BC中点,PH∥AB,
∴ PH = CH = 1/2AB = 1
HN = PN/2 - PH/2 = 7/5
当H在点N的上方时,AM = CN = CH + NH = 1 + 7/5;
当H在点N的下方时,AM = CN = CH - NH = 1 - 7/5
∴ 当DM/AM = 4/5时 ,AM的长为1 + 7/5或1 - 7/5