2. Longitudinal section

[homework] 1, c

2、B

3. The length of the long line ED of the long line EF of the line CD, and the long line cf.

4. Omission

5. AB is slightly connected in the picture, and point B is BC? MN builds roads along AB and BC at point C, and the shortest road length is the reason: What is the distance from point A to point B? Between two points, the line segment is the shortest? From point b to point c? What is the shortest vertical line segment? .

[homework] 6, D.

7、C

8. a

9、3 12

10, (1) 900m1200m

(2) Omission

1 1, of the two schemes, the second scheme is more economical, and the second scheme is not the best. The best scheme is to take point P as the vertical section PN of OB and build roads along PN. The reason is that the vertical section is the shortest, and PN