Zhejiang education printing plate eighth grade first volume mathematics book homework textbook page 9 answer

A: A.

1、DC； BC; ? BCE？ ACB

2. Solution: ∵? ACB=90？ ,

CE is the angular bisector of △ABC,

ECB= 1/2？ ACB=45？ .

∵? CEB= 105？ ,

CED= 180？ =? CEB=75？ ,

∵CD？ AB CDE = 90？ ,

ECD= 180？ -90? -75? = 15? .

3. Solution: Because AD is the midline of △ABC, BD=CD.

And because in △ABD and △ACD,

BD equals the height on the CD,

So S△ABD=S△ACD, so 1/2AB? DF= 1/2AC？ De.

And because AB= 2AC, DE: DF = 2: 1.

4. solution: ∫EF//BC,? AFE=64？ ,

? FEC=？ AFE (two straight lines are parallel and the internal dislocation angles are equal).

∵CE split equally? ACB (known),

ECB= 1/2？ ACB=32？ ,

FEC=？ ECB=32？ .

5. Solution: △ABD circumference =AB+BD+ AD,

△ circumference of △ACD =AC+AD+DC.

AB = 7 cm, AC=5 cm,

AD=AD，BD=DC，

? △ circumference of △ABD-△ circumference of -△ACD -△= 2m.