6. Mrs. Cousins ran to the table, picked up her baby in

　 her arms, ran inside and shut the door.

　　 ☆重点词汇☆

5. For one thing they keep the water clean. Also they

　　 make the tank look much better.

4. She was just about to open the window and shout at the

　 dog to frighten it, when she stopped and stood quite

　 still.

3. It took weeks to clear all the roads and to mend all the

　　 broken electricity lines and telephone lines.

2. If the hurricane had happened during the day-time,

　　 there would have been many more deaths.

1. One woman was lying in bed, awake, listening to the

　 rushing winds.

22．解：(Ⅰ)直角梯形ABCD的面积是

M_底面，

∴ 四棱锥S-ABCD的体积是

　M_底面．　

(Ⅱ)延长BA、CD相交于点E，连结SE，则SE是所求二面角的棱．

∵ AD∥BC，BC = 2AD，

∴ EA = AB = SA，∴ SE⊥SB，

∵ SA⊥面ABCD，得面SEB⊥面EBC，EB是交线，

又BC⊥EB，∴ BC⊥面SEB，故SB是CS在面SEB上的射影，∴ CS⊥SE，

所以∠BSC是所求二面角的平面角．

∵ ，BC =1，BC⊥SB，

∴ tg∠BSC ．

即所求二面角的正切值为．　

21、解：(1)是圆的直径，∴，　 又∽，

∴.

(2)在中，.

∵　　 ∴

又,即，而

∴底面

故三棱锥的体积为

.

20.解：(1)已知EFAB,那么翻折后，显然有PEEF,又PEAE,从而PE面ABC,即PE为四棱锥的高。

四棱锥的底面积 而△BEF与△BDC相似，那么

= ，　 =

则 =63=9

故四棱锥的体积V(x)=Sh=9 = 　(0<x<3)

(2) V’(x)= 3-x²(0<x<3),　 令V’(x)=0得x=6

当x∈(0，6)时，V’(x)>0,V(x)单调递增；x∈(6，3)时V’(x)><0,V(x)单调递减；

因此x=6时， V(x)取得最大值V(x)max= V(6)=12

(3)过F作AC的平行线交AE于点G，连结FG、PG，则EG=6，EF=，GF=PF=，PG=，

19.解：(Ⅰ)如图------ 3分

(Ⅱ)所求多面体体积

．------------------------7分

(Ⅲ)证明：在长方体中，

连结，则．

因为分别为，中点，

所以，

从而．又平面，

所以面．--------------------12分