Đáp án:
$-$
Giải thích các bước giải:
$S=\underbrace{1+2+3\ +\,.\!.\!.+\ 2002+2003+2004}_{\large\rm (2004-1):1+1=2004\ số\ hạng}\\\Rightarrow S=\underbrace{(1+2004)+(2+2003)+(3+2002)\ +\,.\!.\!.}_{\rm\large 1002\ cặp}\\\Rightarrow S=\underbrace{2005+2005+2005\ +\,.\!.\!.}_{\large\rm 1002\ số\ hạng}\\\Rightarrow S=2005.1002=2009010\\\quad\\S=\underbrace{367+361+355\ +\,.\!.\!.+\ 7+1}_{\large \rm 62\ số\ hạng}\\\Rightarrow S=\underbrace{(367+1)+(361+7)\ +\,.\!.\!.}_{\large \rm 31\ cặp}\\\Rightarrow S=\underbrace{368+368\ +\,.\!.\!.}_{\large\rm 31\ số\ hạng}\\\Rightarrow S=368.31=11408$
