Câu 27:
Tính tổng C20000+2C20001+3C20002+...+2001C20002000C_{2000}^0 + 2C_{2000}^1 + 3C_{2000}^2 + ... + 2001C_{2000}^{2000}C20000+2C20001+3C20002+...+2001C20002000?