Find an explicit formula for the geometric sequence − 9   , − 18   , − 36   , − 72 , . . . −9,−18,−36,−72,...minus, 9, comma, minus, 18, comma, minus, 36, comma, minus, 72, comma, point, point, point. c ( n ) = c(n)=c, left parenthesis, n, right parenthesis, equals