1. a+b=18
a-2/b+8=1/3
a) => b+8=3a-6
=>b=3a-6-8=3a-14
=>a+3a-14=18
=>4a=32=> a=8
=>b=18-8=10
2.f(x)=ax+1
a) A(a+1,a+2)€ Gf
=>f(a+1)=a+2
=>a(a+1)+1=a+2
=>a^2+a+1=a+2
=>a^2-1=0=> a^2 =1
=> a= 1 sau a=-1
b)a=-1
=>f(0)+f(1)+f(2)+...+f(2021)=
(-1*0+1)+(-1*1+1)+(-1*2+1)+....+(-1*2021+1)
=1+0-1-2-...-2020
=1-(1+2+3+...+2020)
=1-(2020*2021)/2
