1. (7 points) Point $O$ is the center of square $A B C D$. Find any seven pairwise distinct vectors with endpoints and starting points at points $A, B, C, D, O$, the sum of which is the zero vector. Explain your answer.
Solution. For example, the chain $\overrightarrow{O A}+\overrightarrow{A C}+\overrightarrow{C D}+\overrightarrow{D A}+\overrightarrow{A B}+\overrightarrow{B D}+\overrightarrow{D O}=\overrightarrow{0}$ is suitable. Grading criteria.

- A correct set of seven vectors is provided, or it is proven that their sum equals the zero vector, or it is obvious (based on the vector notations) that the sum equals the zero vector (for example, as indicated in the answer) - 7 points.
- A correct set of seven vectors is provided, but it is not proven that their sum equals the zero vector, - 5 points.
- A set of six vectors is provided, satisfying the other conditions of the problem, - 2 points.
- A set of seven vectors is provided, but there is one pair of equal vectors -1 point.
- A set of vectors is provided, the sum of which does not equal the zero vector, 0 points.