Because precise measurements are needed, a ruler is needed.

The side length of 3 square decimeters is the root of 3 decimeters.

Measure 1 decimeter from the right angle on one side, take this as the center, take 2 decimeters as the radius, and intersect with the other right angle side.

Then the sum of the lengths from the right angle to this intersection point is the root sign 3 points.

In the same way, you can get the root number of 3 decimeters at the other right angle, and you can get 3 square decimeters of paper by making parallel lines.

Principle: Drawing is equivalent to drawing a right triangle, the hypotenuse is 2 decimeters, and the right side is 1 decimeter. According to Pythagorean Theorem, the other side is 3 centimetres.

2.2 5 square decimeter.

Cut a square with a side length of 3 minutes.

Then select 1 decimeter in the same direction from corner to edge on four sides, take a point on each side, and connect four points in turn. The square in the middle has an area of 5 square decimeters.

Principle, after connecting four points in turn, four right triangles are formed in a quadrilateral. The right side of each right triangle is 1 decimeter and 2 decimeters respectively. According to Pythagorean theorem, the length of the hypotenuse is 5 decimeters, so the area of the middle square is 5 square decimeters.