1/10*11+1/11*12+1/12*13+...+1/19*20

0,05

Пошаговое объяснение:

1/(10*11) +1/(11*12) =...= 2/(10*12)

2/(10*12)+1/(12*13) =...= 3/(10*13)

3/(10*13)+1/(13*14) =...= 4/(10*14)

...

а/( 10*(10+а) ) +1/((10+а) *(10+а+1))=...= ( а+1)/( 10*(10+а+1))

...

9/(10*19) +1/(19*20)=...= 10/(10*20)=1/20=0,05