若直线y=kx-2与抛物线y2=8x交于A、B两点，若线段AB的中点的横坐标是2，则|AB|=______．
，k2x2-（4k+8）x+4=0，y2=8x y=kx-2
x1+x2=
=4得k=-1或2，4k+8 k2
当k=-1时，x2-4x+4=0有两个相等的实数根，不合题意，
当k=2时，|AB|=
|x1-x2|=1+k2 5
=(x1+x2)2-4x1x2 5
=216-4
．15
故答案为：2
．15
| 完形填空。先通读下面的短文，掌握其大意，然后在每小题所给的A、B、C、D四个选项中，选出可以填入空白处的最佳选项。 | ||||
| Thousands of years ago, people did not know 1 to make a fire. When it was very 2 at night, they could not sleep well. Sometimes lightning hit (打，击) a 3 and started a fire. At first people were both surprised and 4 to see the fire. They were afraid to go up. Later a few 5 men took some of this fire and made a fire near their homes. They used the fire to keep them warm and to cook food. When they had a fire, they did not let it 6 burning. If it stopped, they could not start it again. Maybe they had to 7 for a long time, sometimes for 8 . Later they 9 how to make a fire with pieces of wood. Now we have matches, we can put them in our pockets and make a fire when we want to make it. But we have to be 10 with fire. | ||||
|