ex 19:
3 intregi si 3/4 inseamna (3*4+3)/4 adica 15/4
2/[k*(k+2)]=1/k - 1/(k+2), adica de exempli 4/(3x5) este (2x2)/(3x(3+2)) = 2/(1/3 -1/5)
deci suma din enunţ devine:
a= 15/4 x 2 x (1/1-1/3+1/3-1/5+...+1/43-1/45)=
=15/2 x (1-1/45) = 15/2 x 44/45 = 22/3
