The Fitting-in of Suzy Khan The first time I saw Suzy Khan, I knew I had to help her．She was really small for her age of 12．The boy in my class often 21 about her and laughed
Our teacher asks us to clean the floor in the morning as our daily . A．routine B．virtue C．scale D．feature
It was at twelve o’clock ___ we finally arrived at the village ____ we spent our happy childhood. A.that; that B.when; that C.when; what D.that; where
如图所示是用厚刻度尺测量木块的长度，正确的测量图是： （ ）
农民将人粪尿收集起来，加水后浇在菜地里，能加快菜苗的生长。请问正常尿液的成分中不含下列何种物质（ ）A．尿素 B．水
氨水是氨气（化学式为NH3）溶于水后形成的，是一种常见的实验药品，它能使无色的酚酞溶液变成红色。 （1）氨气可以用氮气和氢气反应制得，反应的
某同学在“观察人的口腔上皮细胞”实验中，为观察到更清晰的物像，进行了系列操作。下列有关操作与目的对应关系中，正确的是 A．将凹面镜换为平
(14分)如图,在四棱锥中,, ,BC⊥AB,CD⊥AD,BC=CD=PA=a, (Ⅰ)求证:平面PBD⊥平面PAC. (Ⅱ)求四棱锥P-ABCD的体积V;
，则a的取值范围是 . 
<0解析:
变形为
，∵
∴