1a1,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 :)