A:用氢气作气体燃料
B:生活污水直接排入江河
C:汽车尾气任意排放
D:大量使用农药防治虫害
(1)用计算器计算:11-2=____
1111-22=____
111111-222=____
11111111-2222=____
(2)观察题(1)中各式的计算结果,你能发现什么规律?____
(3)试运用发现的规律猜想:1111111111-22222=____ ,并通过计算器验证你的猜想.
— There's somebody at the door. Who ____ it be? Is it the postman?—No. It ______ be him. It's just seven o'clock. It's too early.
如图,∠AOP=∠BOP=15°,PC//OA,PD⊥OA,若PC=10,则PD等于( )