The teacher first took out the disc and said, "Let's draw a circle or take out a round thing and try to measure its circumference." So, we began to discuss. Let's find a way first, and then start the operation. A classmate immediately thought of a way and said, "I have an idea." Make a mark on the disc first, and then scroll from that mark to the right on the ruler to make a mark. The measured length is the circumference of the disc. " An idea came to me at once. I said, "I have an idea, too. Let's wrap a piece of paper around the disc, make a mark, and then measure the length of this paper, which is the circumference of this circle. "
Teacher Shi asked us to report the measured circumference, then measure the diameter of the disc, and then try to divide the circumference of the circle by the diameter. We soon found that the circumference of a circle is related to its diameter, and the circumference of a circle is always more than three times its diameter. At this time, Teacher Shi asked us to open the book and read the words of the wise grandfather on page 9: In fact, the quotient of the circle divided by the diameter of the circle is a fixed number. We call it pi, which is represented by the letter π, and usually takes 3 when calculating. 14, so according to the relationship between circumference and diameter, we can get the calculation method of circumference, that is, the circumference of a circle = diameter × pi ratio, that is, c=πd or c = 2 π r, and the teacher gave many examples after speaking.
We learned how to calculate the circumference of a circle.