The stability of a triangle does not mean that two sides of a triangle are arbitrarily selected, and the non-common endpoints of these two sides are connected by a third side; The third side cannot be stretched or bent; The distance between the two ends is fixed; The angle between the two sides is fixed; These two sides are arbitrary; All three angles of a triangle are fixed, and then the triangle is fixed. Then the non-common endpoints on both sides are connected by more than one edge ∴ The distance between the two ends is not fixed ∴ The included angle between the two sides is not fixed ∴n-polygon (n≥4) is not fixed at each angle, so the N-polygon (n≥4) is unstable.

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