Exercice 32 --- (id : 953)
Suites: Exercice 32
correction
1 U6=U2+(62)rU_6=U_2+(6-2)r     r=U6U24\iff r=\dfrac{U_6-U_2}{4}     r=16+44\iff r=\dfrac{-16+4}{4}     r=3\iff \boxed{r=-3}
U2=U0+2rU_2=U_0+2r     U0=U22r\iff U_0=U_2-2r     U0=4+6=2\iff \boxed{U_0=-4+6=2}
2 Un=U0+nrU_n=U_0+nr     Un=23n\iff \boxed{U_n=2-3n}
3 S=U3+U4+...+U14S=U_3+U_4+...+U_{14} =143+12(U3+U14)=\dfrac{14-3+1}{2}(U_3+U_{14}) =6(23×3+23×14)=6(2-3\times3+2-3\times14) =6(4942)=282=6(4-9-42)=-282
4
a Sn=n+12(U0+Un)S_n=\dfrac{n+1}{2}(U_0+U_n) =n+12(2+(23n))=\dfrac{n+1}{2}(2+(2-3n)) =(n+1)(43n)2=\dfrac{(n+1)(4-3n)}{2} =4n3n2+43n2=\dfrac{4n-3n^2+4-3n}{2} =3n2+n+42=\dfrac{-3n^2+n+4}{2}
b Sn=143S_n=-143     3n2+n+42=143\iff \dfrac{-3n^2+n+4}{2}=-143     3n2+n+4=286\iff -3n^2+n+4=-286     3n2n290=0\iff 3n^2-n-290=0 or Δ=3481\Delta=3481 et Δ=59\sqrt \Delta=59 donc n=1+596=10n=\dfrac{1+59}{6}=10 ou n=1596Nn=\dfrac{1-59}{6}\notin \Bbb N d'où
Sn=143S_n=-143     n=10\iff n=10