Zhang Lei tracked the size of the bear population in a nature reserve. The first year, there were $1000$ bears. Sadly, the population lost $10\%$ of its size each year. Let $f(n)$ be the number of bears in the reserve in the $n^\text{th}$ year since Zhang Lei started tracking it. **$f$ is a sequence. What kind of sequence is it?** [[☃ radio 1]] **Complete the recursive formula for $f(n)$.** $f(1)=$ [[☃ input-number 3]] $f(n)=f(n-1)$[[☃ dropdown 1]][[☃ input-number 4]]