The calculation method of triangle area recorded in China's ancient mathematical masterpiece "Nine Chapters Arithmetic" is "half width multiplied by follow", where "width" refers to the bottom of the triangle;

"Half width times positive follower". "Width" refers to the bottom of the triangle, and the follower refers to the height on the bottom. The area of a triangle is equal to half the product of the height and the length of the base.

Using the method of "making up for the loss with profit", turn the lower triangle upside down to make up for the upper one and get a rectangle. Then the length of the converted rectangle is the height of the triangle, the width is half of the bottom of the triangle, and the area of the rectangle is equal to the area of the triangle.