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let-A-n-0-ne-x-dx-with-n-2-1-calculate-A-n-2-find-nature-of-n-2-A-n-3-study-the-convergence-of-1-A-n-and-1-A-n-2-




Question Number 41410 by maxmathsup by imad last updated on 06/Aug/18
let A_n =∫_0 ^∞   [ne^(−x) ]dx  with n≥2  1) calculate A_n   2) find nature of Σ_(n≥2)    A_n   3) study the convergence of  Σ (1/A_n )  and Σ (1/A_n ^2 )
letAn=0[nex]dxwithn21)calculateAn2)findnatureofn2An3)studytheconvergenceofΣ1AnandΣ1An2
Commented by maxmathsup by imad last updated on 07/Aug/18
1) changement n e^(−x)  =t give e^(−x)  =(t/n) ⇒e^x  =(n/t) ⇒x =ln(n)−ln(t) ⇒  dx =−(dt/t) ⇒ A_n =− ∫_n ^0    [t]  (dt/t) = ∫_0 ^n   (([t])/t) dt =Σ_(k=0) ^(n−1)   ∫_k ^(k+1)  (k/t) dt  = Σ_(k=0) ^(n−1)  k {ln(k+1)−ln(k)} =Σ_(k=1) ^(n−1) k{ln(k+1)−ln(k)}⇒  A_n =Σ_(k=1) ^(n−1)  k ln(1+(1/k))
1)changementnex=tgiveex=tnex=ntx=ln(n)ln(t)dx=dttAn=n0[t]dtt=0n[t]tdt=k=0n1kk+1ktdt=k=0n1k{ln(k+1)ln(k)}=k=1n1k{ln(k+1)ln(k)}An=k=1n1kln(1+1k)
Commented by maxmathsup by imad last updated on 07/Aug/18
2) we have ln^′ (1+x) =(1/(1+x))=1−x  +x^2  −... if ∣x∣<1 ⇒  ln(1+x) =x−(x^2 /2) +(x^3 /3) −... ⇒ ln(1+x)≥x−(x^2 /2) ⇒ln(1+(1/(k )))≥(1/k) −(1/(2k^2 )) ⇒  ∀ k∈[[0,n−1]]  kln(1+(1/k))≥1 −(1/(2k)) ⇒Σ_(k=1) ^(n−1) k ln(1+(1/k)) ≥Σ_(k=1) ^(n−1) (1−(1/(2k))) ⇒  A_n ≥  n−1 −(1/2) Σ_(k=1) ^(n−1)  (1/k) ⇒ A_n ≥ n−1 −(1/2) H_(n−1)    but  H_(n−1) =ln(n−1) +γ +o((1/n)) ⇒n−1−(1/2) H_(n−1) =n−1−ln(√(n−1)) −(γ/2) +o((1/n))  but lim_(n→+∞) n−1−ln(√(n−1)) =+∞ ⇒ A_n  →+∞ ⇒ Σ A_n  diverges
2)wehaveln(1+x)=11+x=1x+x2ifx∣<1ln(1+x)=xx22+x33ln(1+x)xx22ln(1+1k)1k12k2k[[0,n1]]kln(1+1k)112kk=1n1kln(1+1k)k=1n1(112k)Ann112k=1n11kAnn112Hn1butHn1=ln(n1)+γ+o(1n)n112Hn1=n1lnn1γ2+o(1n)butlimn+n1lnn1=+An+ΣAndiverges

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