Today we used these old calendars to make these beautiful barbolas .Pay attention to the trash. Who can tell me which tr今天，我们知道了旧挂历原来也可以做成这么漂亮的剪贴画，我们千万不要小瞧了生活中的废弃品，你还知道哪些废品可以再利用呢？
They can be expressed as two parabolas and four empiric V_p-V_s formulas.它们可以拟合表达成两条抛物线,四个V_p、V_s互推公式。