AN*BN= 1/(N +N)

(AN-BN) squared =(AN+BN) squared -4AN*BN=(2N+ 1) squared /(N squared +N) squared -4/(N squared +N)

=[(2N+ 1) square -4(N square +N)]/(N square +N) square

= 1/(N +N) square

The line length anbn =1/(n+n) =1[n (n+1)] =1/n-1(n+1).

Then the formula =1-1/2+1/2-1/3+* * *+1/2009-1/20.

Option d