Final answer:
Jacob Riis's purpose in writing 'How the Other Half Lives' was to inform about social issues and generate public support for change. His audience was the general public.
Step-by-step explanation:
Jacob Riis's purpose in writing 'How the Other Half Lives' was to inform about social issues. His audience was the general public. Riis sought to expose the terrible living conditions of the urban poor, particularly in tenement slums, through his photographs and writings. He aimed to generate public awareness and support for change, leading to improvements in tenements and the enactment of new laws.