The four areas formed by the horizontal axis and the vertical axis in the coordinate axis are divided into four quadrants with the origin as the center and the X axis and the Y axis as the dividing lines. The upper right is called the first quadrant (+,+), the upper left is called the second quadrant (-,+), the lower left is called the third quadrant (-,-), and the lower right is called the fourth quadrant (+,-).

Introduction of the first quadrant

The abscissa of the point in the first quadrant is greater than 0, and the ordinate is also greater than 0.

Add a vertical straight line at the 0 o'clock position of the number axis to form a Cartesian coordinate diagram. The area in the upper right corner is called the first quadrant, the area in the upper left corner is called the second quadrant, the area below the second quadrant is called the third quadrant, and then the area below the first quadrant, that is, the right side of the third quadrant is called the fourth quadrant. At this time, the concept of angle is the angle at which the light in the positive direction of the X axis (the one on the right) rotates counterclockwise around the O point. It can go around many times, for example, it returns to the positive direction of the X axis twice, so the angle is 720 degrees.

The first quadrant: (positive+,+positive), the abscissa and ordinate have the same sign, and it is denoted as xy>0.