16.50 17.5/3 18.6n-2 19 . n？ /2-n/2+ 1 20.5

2 1.( 1)d=( 16-25)/3=-3

an=25-3(n- 1)=-3n+28

(2)-3n+2828/3

Starting from project 10, n is less than 0.

③a 1+a3+a5+...+a 19 = 10×(25+25-9×6)÷2 =-20