The length of one side of a triangle and the diameter of a circle.

When you get the topic, you should form an image in your mind: an equilateral triangle with a circle inside it. Then you can work out all the values. Don't look at the topic. Let's mentally calculate the relationship between the radius of a circle and one side of an equilateral triangle: 1: √ 3. Other relationships, such as the relationship between the area of a triangle and a circle, can also be calculated. At this time, let's look at the two limbs of the topic: the side length of a triangle and the diameter of a circle (note that the diameter is not a radius), and their ratio is √ 3.