Bonsoir,
[tex]a)\ u_0=750\\b)\ u_1=0.6*u_0+100=550\\\\\boxed{u_{n+1}=0.6*u_n+100}\\c)\\On\ pose \ v_n=u_n-250\\v_{n+1}=u_{n+1}-250\\=0.6*u_n+100-250\\=0.6*u_n-150\\=0.6*(u_n-250)\\=0.6*v_n\\v_0=u_0-250=750-250=500\\\\v_n=500*0.6^n\\\\\boxed{u_n=500*0.6^n+250}\\\\d)\\u_{25}500*0.6^{25}+250=250.0014215....\\\\Au\ 01/02/2021,\ il\ y\ aura\ 250\ abonn\'es.\\[/tex]