The equilateral triangle has the largest area for a triangle with a certain perimeter; For a triangle with a certain area, the perimeter of an equilateral triangle is the smallest.

Point to straight line, the shortest distance;

Between two points, the line segment is the shortest;

A quadrilateral with a certain perimeter, the square area is the largest; A quadrilateral with a certain area has the smallest perimeter.

Plane figure with fixed perimeter and the largest area of circle; When the area is fixed, the circumference of the circle is the smallest.