S_{n} - q S_{n} = a^{1} + a_{2} + \dots + a_{n} - (a_{2} + \dots + a_{n + 1}) = = a_{1} - a_{n + 1}

${S_n - qS_n = a^1 + a_2 + \ldots + a_n - (a_2 + \ldots + a_{n+1}) = \\=a_1 - a_{n + 1}}$

Error