②“ $-\frac{1}{2}<x<0$ ”是“2x²﹣5x﹣3＜0”必要不充分条件；

③“若xy=0，则x，y中至少有一个为0”的否命题是真命题；

④曲线 $\frac{x^2}{25}+\frac{y^2}{9}=1$ 与曲线 $\frac{x^2}{25-k}+\frac{y^2}{9-k}=1\left(9<k<25\right)$ 有相同的焦点；

⑤过点（1，3）且与抛物线y²=4x相切的直线有且只有一条．

其中是真命题的有：____（把你认为正确命题的序号都填上）

《汉乐府·长歌行》中打比方的诗句是（  ）。

当实验室光线较暗时，显微镜的调节宜为 

    Have you ever heard an echo of your voice?
    An echo is a reflected(反射)sound. Stand in a very large, empty hall with nothing on the walls. Then shout. The sounds you make will hit the walls and come back to you. The walls reflect the sounds. These reflected sounds are called “echoes”.
    We can use reflected sound to make sound travel farther. Hold a watch about fifty centimeters from your ear. You cannot hear it. The sound of the watch travels a short way in all directions. It does not travel as far as your ear. You can make it travel to your ear by using a roll of cardboard(纸板筒).
    Make the roll of cardboard about fifty centimeters long. Put one end over your ear and hold the watch at the other. Now you can hear the watch because the sound has not gone in all directions. It is reflected down the roll to your ear.
    A megaphone(喇叭筒)works in the same way. It makes sound travel farther. It reflects the sound into a beam which can travel a long way. In the ship, the sailor uses the megaphone to make his voice travel. Without the megaphone, his voice cannot travel far.
    We use reflected sound in many ways. We use it to measure the depth of the sea, for example. Among the animals, bats and whales use reflected sound to tell them where they are and what things are near them.