Câu 20:
Tính tổng S=520C200−519C201+518C202−...+C2020S={{5}^{20}}C_{20}^{0}-{{5}^{19}}C_{20}^{1}+{{5}^{18}}C_{20}^{2}-...+C_{20}^{20}S=520C200−519C201+518C202−...+C2020