Question

1/3*7+1/7*11+1/11*15+1/11*19+1/19*23

Answer 1

Решение задания прилагаю

Answer 2

1/n(n + 4) = 1/4*4/n(n + 4) = 1/4*(n + 4 - n)/n(n + 4) =  1/4*(1/n - 1/(n + 4))

1/3*7+1/7*11+1/11*15+1/11*19+1/19*23 = 1/4*(1/3 - 1/7 + 1/7 - 1/11 + 1/11 - 1/15 + 1/15 - 1/19 + 1/19 - 1/23) = 1/4*(1/3 - 1/23) = 1/4*20/69 = 5/69