The diameter of the circular signboard is 40 cm. At present, there is a rectangular iron plate with a length of 12dm and a width of 9dm. This iron plate is at most: 40 cm =4 decimeters; 12÷4×9÷4≈3×2=6 (blocks), and 6 signboards can be made at most.

Extended data

Names of the parts of a circle

1, center of the circle: after drawing a circle with a compass, the point where the needle tip is fixed is the center of the circle, usually represented by the letter O, and the center of the circle determines the position of the circle.

2. Radius: The line segment connecting the center of the circle and any point on the circle is called radius. Generally, it is represented by the letter R. If the two feet of a compass are separated, the distance between the two feet is the radius of the circle.

3. Diameter: The line segment whose two ends pass through the center of the circle is called diameter. Generally, the letter D indicates that the diameter is the longest line segment in a circle.

The main characteristics of a circle

1, in the same circle or equal circle, there are countless radii and countless diameters. All radii are equal and all diameters are equal. 2. In the same or equal circle, the length of the diameter is twice that of the radius, and the length of the radius is 1/2 of the diameter. Expressed in letters: d=2r or r=d/2.

3. If a graph is folded in half along a straight line, the graphs on both sides can completely overlap, and this graph is axisymmetric. A circle is an axisymmetric figure with countless axes of symmetry.

The Significance of Pi and the Circumference Formula of Circle

1. Pi experiment: make a mark on the circular paper, align it with the scale of ruler 0, and roll it on the ruler once to find out the circumference of the circle. It is found that the ratio of the circumference to the diameter of a circle is a fixed number (TT).

2. Pi: The ratio of the circumference to the diameter of any circle is a fixed number, which we call Pi. Represented by letters.

The circumference of a circle is always more than 3 times its diameter, and this ratio is a fixed number. πt is an infinite cyclic decimal. In the calculation, T3. 14 is generally taken.

4. When judging, the ratio of circumference to diameter of a circle is t times, not 3. 14 times. The first person in the world to calculate pi was China mathematician Zu Chongzhi.