حل التمرين 64 ص 22
S = 1/2 +1/6 +1/12 +1/20 + 1/30
S = 30/60 + 10/60 + 5/60 +3/60 + 2/60
S = 50/60
S = 5/6

A = 1/n – 1/ n + 1
(A = n +1 /n(n+1) - n/n(n + 1
(A = 1 / n(n+1
A(1) = 1/ 1(1+1) =1/2
A(2) = 1 /2(2+1) =1/6
A(3) = 1 /3(3+1) = 1/12
A(4) = 1/4(4+1) = 1/20
A(5) =1 /5(5+1) = 1/30

A(n) = 1/n – 1 /n+1
A(1) = 1 - 1/2
A(2) = 1/2 - 1/3
A (3) = 1/3 - 1/4
A(4) = 1/4 - 1/5
A (5) = 1/5 – 1/6

بالجمع نجد
A(1)+A(2) +A(3) +A(4) +A(5) = 1 - 1/6 =6/6 – 1/6 = 5/6
S = 1/2 +1/6 +1/12 +1/20 + 1/30
S = 1 – 5/6