题目

Plants are very important living things. Life could not go on if there were no plants. This is because plants can make food from air,water and sunlight. Animals and man cannot make food from air,water and sunlight. Animals get their food by eating plants and other animals.Therefore animals and man need plants in order to live.This is why we find that there are so many plants around us. If you look carefully at the plants around you,you will find that there are two kinds of plants:flowering plants and non-flowering plants. Flowering plants can make seeds（种子）. The seeds are protected by类比正三角形的内切圆切于三边的中点，得出正四面体的内切球切于各面正三角形的位置是