CuSO₄溶液对过氧化氢的分解有催化作用．取溶质质量分数为8.5%的过氧化氢溶液20g倒入小烧杯中，向其中加入溶质质量为0.75g 的CuSO₄溶液10.8g，使过氧化氢完全分解．试计算：

（1）产生氧气的质量；

（2）反应后所得溶液中CuSO₄的质量分数．

设圆x²+y²=12与抛物线x²=4y相交于A，B两点，F为抛物线的焦点，若过点F且斜率为1的直线l与抛物线和圆交于四个不同的点，从左至右依次为P₁，P₂，P₃，P₄，则|P₁P₂|+|P₃P₄|的值____，若直线m与抛物线相交于M，N两点，且与圆相切，切点D在劣弧 $\widehat{AB}$ 上，则|MF|+|NF|的取值范围是____．

两个面积相等的圆，它们的周长一定相等。

     All students need to have good study habits. When you have good study habits, you will learn things quickly and also remember them easily. Do you like to study in the living room? This is not a good place, because it is usually too noisy. You need to study in a quiet place, like your bedroom A quiet place will help you only to think about one thing. When you study, do not think about other things at the same time. Only think about your homework. If you do so, you will do your homework more quickly, and you will make fewer mistakes.
    Good study habits are very important. If you do not have them, try to learn them. If your studyhabitsare already good. Try to make them better.

如图所示，O₁为皮带的主动轮的轴心，轮半径为r₁，O₂为从动轮的轴心，轮半径为r₂，r₃为固定在从动轮上的小轮半径．已知r₂＝2r₁，r₃＝1.5r₁.A，B和C分别是3个轮边缘上的点，质点A，B，C的向心加速度之比是(    )