Răspuns:
1/n(n+1) =1/n - 1/n+1
=>1-1/2 +1/2 -1/3 + 1/3...1/n-1 -1/n=1919/1920
1-1/n=1919/1920
n-1/n =1919/1920
1920n-1920=1919n
n=1920
Explicație pas cu pas:
1/1×2=1-1/2
1/2×3=1/2-1/3
1/3×4=1/3-1/4
1/(n-1)×b=1/(n-1)-1/n
........................................
∑=1-1/n
1-1/n=1919/1920 (n-1)/n=1919/1920
1920n-1920=1919n 1920n-1919n=1920
