Shanna writes the formula f(x + 1) = 2. 5f(x) when f(1) = 2 to represent this sequence: 2, 5, 12. 5, 31. 25, … which error did shanna make? she used the incorrect common ratio. She used the incorrect initial value. She should have multiplied by f(x) rather then have it as an exponent. She treated the sequence as geometric instead of arithmetic.