f(x+1)=(-1)^x+1x – 2f(x); x belong to N.

f(1)=f(1986)

Compute the sum of the digit of the number (f(1)+f(2)+f(3)+...........+f(1985))

Answer is 331. Find f (2), f (3)....till f (1986).Then add all the terms and arrange them by takin similar terms to left side frim right side.This will form like 3(f 1+f2+f3+....f1985)=993.