S=1−2+3−4+...+2005−2006S=1−2+3−4+...+2005−2006

⇔SS = (1−21−2) + (3−43−4) + ..... + (2005−20062005−2006)

⇔SS = −1−1 + (−1−1) + .....+ (−1−1)

Dãy trên có : [(2006−12006−1) : 1+11+1] : 2=10032=1003 (Cặp có giá trị =−1=−1)

⇔SS= −1.1003−1.1003

⇔SS =−1003−1003

   Vậy SS =−1003−1003