H.P

a

a1,a2,a3...... r in hp

1/a1,1/a2........... r in ap let d diffrnce be d

a1a2+a2a3+a3a4...........a(n-1)an

=1/d(a1-a2+a2-a3+a3........... +a(n-1)-an)
=1/d(a1-an)..................telescopic sum banta hai

=(n-1)/(n-1) *(a1-an)

=(n-1)(a1-an)/(1/an-1/a1) =(n-1) a1an

mek o bhi answer ni aaya tha wen i had solvd 4 d first tym so sol dekh liya tha :)

Arrey just take n=3, Hence it becomes a1a2+a2a3
As it is hp,a2=(2a1a3)/a1+a3
Hence simplifying we get 2a1a3 which is a