Question

Acest exercițiu va rog.

Answer 1

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

Answer 2

Răspuns:

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

n=1920