i3) a(k)= a(1) + (k-1)*r
r= rația progresiei aritmetice
a(5)=a(1) + (5-1)*3= 2 + 4*3= 2 + 12=14
i4) r=5; a(8)=70; a(1)=?
a(8)= a(1) + (8-1)*r --> 70=a(1) +7*5 --> a(1)=70-35=35
i5) r=4; a(6)= -13; a(10)=?
a(6)=a(1)+(6-1)*r --> -13=a(1)+5*4 --> a (1)= -13-20= -33;
a(10)= a(1)+(10-1)*r --> a(10)= -33+9*4= -33+36=3
:D