Répondre :
Vn = 1 / ( Un - 1 )
Vn+1= 1/(Un+1 - 1)
= 1/ (( 4 Un - 1 ) / ( Un + 2 )) -1
= (Un +2 ) / 3(Un - 1)
Vn+1 - Vn = ((Un +2 ) / 3(Un - 1)) - 3/3(Un - 1) = 1/3
Vn+1= 1/(Un+1 - 1)
= 1/ (( 4 Un - 1 ) / ( Un + 2 )) -1
= (Un +2 ) / 3(Un - 1)
Vn+1 - Vn = ((Un +2 ) / 3(Un - 1)) - 3/3(Un - 1) = 1/3