Let first term = a and common difference = d
Then, a1 = a, a2 =a+d, a3= a+2d.....
1/a1.a2 + 1/a2.a3 = 1/a(a+d) + 1/((a+d)(a+2d) = (a+2d+a)/a(a+d)(a+2d) = 2/a(a+2d) = 2/a1.a3
now 2/a1.a3 + 1/a3.a4 = 2/a(a+2d) +1/(a+2d)(a+3d) = (2a+6d+a) /a(a+2d)(a+3d) = 3/a(a+3d) = 3/a1.a4
So if in denominator we have an then in numerator we will be having (n-1).
Continuing this the sum of given expression will be = (n-1) /a1.an