Răspuns:
Explicație pas cu pas:
a₅ = 21 ; a₉ = 37
a₇ = (a₅+a₉)/2 = (37+21)/2 = 58/9 = 29
a₆ = (a₅+a₇)/2 = (21+29)/2 = 50/2 = 25
r = a₆-a₅ = 25-21 = 4
a₁ = a₅-4r = 21-4·4 = 21-16 = 5
a₁ = 5 ; a₀ = a₁-r = 5-4 => a₀= 1
a₁₀ = a₁+9r = 5+9·4 = 5+36 = > a₁₀ = 41
S₁₂ = a₁+a₂+a₃+.....+a₁₂ = a₁+a₁+r+a₁+2r+......+a₁+11r = 12a₁+r(1+2+...+11) =>
S₁₂ = 12·5 + 4·(1+11)·11:2 = 60+4·6·11 = 60+264 = 324
