Similarly, you can compare Sina, so it's easier to understand!
Now answer your second question: What is it for?
First of all, we express the numbers we have learned before (such as x, a, b, etc.). ) Correspond one to one on a line, mark the direction and specify the unit length, so that we can learn numbers on a line. In other words, we can express all the numbers with the same characteristics in one line! This has the following two meanings: 1. We can use geometric figures (points, lines, faces, etc.). ) to represent quantities with the same characteristics, thus making them more systematic and orderly. Take Sina as an example. In junior high school, the sine value representing a specific angle. We can find that the sine of an angle is one, but find its characteristics and put them in a rectangular coordinate system. Through accurate drawing, the sine value of an angle can be found faster through graphics (of course, this is not very accurate, but it was very exciting in the era around 18 century); 2. Representing algebra with geometric figures lays the foundation for the combination of numbers and shapes, and makes the invariant numbers move on the geometric figures (I believe that these moving points are not mentioned in pure algebra and pure geometry), which provides good ideas and methods for many problems that are difficult to solve in pure algebra and pure geometry, which is why the teacher repeatedly emphasizes the combination of numbers and shapes!
Secondly, the trigonometric function with sinA was first introduced to study triangles and circles, and its formulas can be transformed with each other, which is incomparable to any other function or formula. With the development of production, it has been widely used in other disciplines and life. When you enter the university and take more courses, you will naturally understand!
This is my understanding, I hope it will help you!