aaaaa.....AP...

try to change 1a₁a₂ in the form of k(1a₁ - 1a₂) and then solve.....

1a₁a₂ = k(1a₁ - 1a₂)

→ 1 = k(a₂ - a₁)
→ 1 = kd, d: common difference
→ k = 1d

Hence, we can write all the terms as:
1a₁a₂ = 1d(1a₁ - 1a₂) ...................(1)
1a₂a₃ = 1d(1a₂ - 1a₃) ......................(2)

and so on, till

1a_n-1a_n = 1d(1a_n-1 - 1a_n)....................(n)

Now add all these to get the required sum