Răspuns:
Explicație pas cu pas:
1) a₁ = 6 ; r = -3
a₂₄ = a₁+23r = 6+23·(-3) = 6-69 = -63
a₇ = a₁+6r = 6+6·(-3) = 6-18 = -12
a₁₂ = a₁+11r = 6+11·(-3) = 6-33 = -27
2) x-1 ; x/2 ; 3+x <=>
x/2 = (x-1+3+x)/2 => x = 2x+2 => x = -2
=> -3 ; -1 ; 1 ; r = 2
3) a₁ = -4 ; r = 2
S₂₄ = a₁+a₂+a₃+.....+a₂₄ = a₁+a₁+r + a₁+2r +.....+a₁+23r
S₂₄ = 24a₁+r(1+2+3+....+23) = >
S₂₄ = 24·(-4) +2·(1+23)·23:2
S₄ = -96 + 552
S₂₄ = 456
4) a₂₉ = 123 ; a₂₄ = 67
a₂₉ = a₂₄+ 5r = 67+5r = 123 =>
5r = 123-67 = 56 => r = 56:5 => r = 11,2
a₁ = a₂₄ - 23r = 67- 23·11,2 = 67 - 257,6 => a₁ = - 190,6
