2. Pythagorean theorem is a basic geometric theorem. The sum of the squares of the sides of two right angles (namely "hook" and "strand") of a right triangle is equal to the square of the side length of the hypotenuse (namely "chord"). In 1 1 century BC, mathematicians in the Zhou Dynasty put forward "Gou San, Gu Si and Xian Wu". When two right-angled sides of a right-angled triangle are 3 (short side) and 4 (long side) respectively, the radius angle (chord) is 5. In the future, people will simply describe this fact as "hooking three strands, four strings and five".
3. First, we measure a 4-meter-long line segment and mark it. Then we make a 3-meter-long line segment approximately perpendicular to the 4-meter-long line segment and mark it. Finally, we measure the distance between the two marks at the two ends of the 4-meter-long and 3-meter-long line segment with a one-meter ruler, so that the angle formed by the 4-meter-long and 3-meter-long line segments is 90 degrees (right angle).
Fourth, as shown in the figure below.