For example, 2/1* (1+2) =1-1/3.

Another example is 2/3 * (3+2) =1/3-1/5.

But in this problem, the molecule is 1 instead of 2, so multiply the original formula by 2 and divide it by 2.

Solution: The original formula = [11* 3+1/2 * 4+1/3 * 5+1/4 * 6 ...+1/kloc.

=[2/ 1*3+2/2*4+2/3*5+ 1/4*6.....+2/ 18*20]/2

=[ 1- 1/3+ 1/2- 1/4+ 1/3- 1/5+ 1/4- 1/6+.....+ 1/ 18- 1/20]/2

=[ 1- 1/ 19+ 1/2- 1/20]/2

=( 18/ 19+9/20)/2

=9/ 19+9/40

=53 1/760