The mathematical addition in the second volume of the first day of junior high school is the carry addition within 20. A circle is to reflect the meaning of the ten-point method.

Take 9+4 as an example. See 9. It is considered that 1, 9 and 1 constitute 10. To calculate 9+4 with ten, you have to draw a circle and circle 9 and 1. This circle divides the second addend 4 into 1 and 3, so the sum of 9+4 is 1 and 3 in the circle.