1 . a₁ = 1 a₅ =a₁ + 4r = 1 + 4r =13
4r = 12 ; r = 3
a₂₀₀₈ = a₁ + 2007· r = 1 + 2007 · 3 = 6022
2 , a₁ + 9r - ( a₁ + r) = 16 ; a₁ + 9r - a₁ -r = 16 ; 8r = 16 ; r = 2
3. b₁ =2
b₂ = b₁·q ; 2q = 6 ; q= 3
b₅ = b₁·q⁴= 2 ·3⁴ = 162
1+5+9+13+17+21+25 = ( 1 + 25) ·7 / 2 = 26 ·7 / 2 = 13 ·7 = 91
progresie aritmetica S ₇ = ( a₁ + a₇ ) · 7 / 2