004 Sample Final A, Problem 7

Given a sequence $10,7,4,1,\ldots$ use formulae on the back page to compute Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle S_{20}}