Triangle rule: Triangle rule refers to the synthesis of two forces (or any other vector). When the starting point of one force is moved to the end point of another force, the resultant force is the force from the starting point of the stationary force to the end point of the moving force.

The law of parallelogram is: the law of parallelogram is the law of physics. When two forces are combined, the line segment representing the two forces is regarded as the adjacent side to make a parallelogram, and the diagonal between the two adjacent sides represents the magnitude and direction of the resultant force, which is called the parallelogram rule.

The characteristics of the triangle:

1. The center of a triangle is the center of the inscribed circle of the triangle, that is, the intersection point of the bisectors of the three angles of the triangle, and its distances to the three sides of the triangle are equal.

2. The outer center of the triangle is the center of the circumscribed circle of the triangle, that is, the intersection of the perpendicular lines of the three sides of the triangle, and the distances from it to the three vertices of the triangle are equal.

3. The center of gravity of a triangle is the intersection of the midlines of three sides of the triangle, and its distance from the vertex is twice as long as that from the midpoint of the opposite side.

4. The vertical center of a triangle is the intersection of the heights of three sides of the triangle, which can form many right-angled triangles.

5. The center of the side of a triangle is the intersection of an inner bisector and its two non-adjacent outer bisectors, and its distances to the three sides are equal.