Răspuns:
Explicație pas cu pas:
Un mod de calcul:
91. 1/(1*3) = 1/2(1/1 - 1/3) ;1/(2*4) = 1/2(1/2-1/4) ; ...
an = 1/2(1-1/3+1/2-1/4+1/3-1/5...-1/(n+2)
an = 1/2(1+1/2 -1/(n+2) = (3n+4)(4(n+2))
lim an = 3/4
92. 1/(1*4) = 1/3(1-1/4) ; 1/(4*7) = 1/3(1/4 - 1/7) ....
Factor comun 1/3 etc
