题干

以双曲线16
x
2
-
y
2
9
=1
的右顶点为焦点的抛物线的标准方程为  (    )

A:y2=16x

B:y2=-16x

C:y2=8x

D:y2=-8x

上一题 下一题 0.0难度 选择题 更新时间:2013-02-27 09:37:20

答案(点此获取答案解析)

A

同类题3

阅读下文,回答问题

小心!二维码暗藏“杀机”

    ①咔嚓一拍,信息自来。一个个小小的黑白方块——二维码,就像一把把钥匙,带我们走进不同的世界。不知不觉中,我们已经习惯这种轻松获取资讯的方式了。但你有没有想过,手中轻易得来的“钥匙”究竟是“金钥匙”,还是“黑钥匙”?

    ②二维码是在一维条码的基础上扩展出来的,它使用黑白矩形图案表示二进制数据,被设备扫描后,可获取其中所包含的信息。以前常用的一维条码只有宽度记载着数据,而二维条码的长度、宽度均记载着数据。因此,相比前者,二维码的信息容量更大,最多记录____个大写字母_____个数字或___多个汉字,是普通条码的几十倍。而且,除文字数字外,它还能存储图片、声音、指纹等信息。也正因如此,二维码才被称为不含电子芯片的廉价存储器。

    ③如今我们常见QR条形码(Quick Response,译为“快速反应”)是二维码家族中的一员,可被智能手机扫描,便于用户直接访问某个网站而无需键入网址。这就给黑客和骗子以可乘之机。他们只需付出很少的成本,利用网上随处都能下载到的二维码生成软件,就能印制出大量二维码贴纸,把它们贴在各地的公共广告上,覆盖原本由企业提供的二维码,给人造成误导。而这些假二维码所含的网址,最终将受害者引向恶意网站。由于二维码是由抽象的矩阵图形组成的,人们无法一眼辨认出它的真伪,再加之其过于便捷,易使人放松警惕,因此骗子更容易得手。

    ④所以,对让我们又爱又恨的二维码,大家还是多留个心眼儿的好。

(摘自《青年参考》有删节)

同类题4

阅读下面短文, 然后根据短文内容补全答题卡上对应表格中所缺失的信息,每空限1词。

    The "green" car has become popular in recent years. A green car is an environment friendly car. Today, most cars are powered (驱动) by petrol (汽油). They make our life easier. But cars have also made problems for the environment.

    There are a few new types of cars that use no petrol or a little petrol.

    Electric cars:This kind of car uses a battery (电池) for power. When you're not driving it, just plug it in like a mobile phone to power up the battery. They don't make exhaust (排放物). But car makers still can't make a battery that lasts for more than a few hours. That's why electric cars aren't very popular.

    Fuel cell cars (燃料电池车) : Fuel cell cars use hydrogen (氢) and oxygen (氧) to make power. The only exhaust they produce is clean water. But fuel cell cars are still much too expensive to make.

    Solar cars : This kind of car has large panels (板) on the top that take power from the sunshine. The car holds that power in a battery. Solar cars might be the cleanest of all. But no one uses them, because solar panels can't get enough power from the sun to make a car go for a long time. Also, they are very slow.

    Hybrid cars(油电混合车): This kind of car is the most exciting kind, because it is very clean and not too expensive. The hybrid car is very smart: Half the time, it uses petrol, and half the time it uses a battery. So, when the petrol engine is working, it puts some of its power back into the battery. It uses much less petrol than other cars. It also makes less pollution for the environment.

New types of

cars

Powered by

Problems

Electric cars

a ____

The battery doesn't last for more than a few hours.

Fuel cell car

hydrogen and oxygen

Fuel cell cars are much too ____to make.

Solar cars

____

Solar panels can't get enough power from the sun to make a car go for a long time. This type of car is too slow.

Hybrid cars

____and a battery

It makes less ____for the environment.