The fish population in a local lake is 10,000. The population increase by half every year. Which function represents the fish population after x years?
1. f(x) = 1.5(10,000)^x
2. 10,000(-1.5)^x
3. None
4. f(x) = 10,000(0.5)^x
5. f(x) = 10,000 (1.5)^x