$\\frac{1}{2d} \\left( \\frac{1}{a_1 a_2} - \\frac{1}{a_{n-1} a_n} \\right)$

$\\frac{1}{d} \\left( \\frac{1}{a_1 a_2} - \\frac{1}{a_{n-1} a_n} \\right)$

$\\frac{1}{2d} \\left( \\frac{1}{a_1 a_2} + \\frac{1}{a_{n-1} a_n} \\right)$

$\\frac{n-2}{a_1 a_2 a_n}$