let give a_n = Π_(k=1) ^n cos( (π/((k+2)!))) and
b_n =Π_(k=1) ^n sin((π/((k+2)!)))
1) find a equivalent foe a_n and b_n
2) find a equivalent of a_n .b_n .
1) prove that if f is decreasing function we have
∫_n ^(n+1) f(t)dt <f(n)< ∫_(n−1) ^n f(t) dt .
2) let put S_n = Σ_(k=1) ^n^2 (1/(2(√k))) .calculate [S_n ].