a6=2 a19=15 Obliczenia do a1 i r!!! a6=a1+5r a19=a1+18r a1+5r=2/*(-1) a1+18r=15 -a1-5r=-2 a1+18r=15 13r=13/:13 r=1 a1+5*1=2 r=1 a1=2-5 r=1 a1=-3 r=1 Wzór ogólny a47=a1+46r a47=-3+46*1=43 S47=-3+43/2*47=40/2*47=20*47=940 ?a15 daje 19 wyrazów. Proszę Bardzo
Dane: a₆ = 2 a₁₉ = 15 a) a₁ + 5r = 2 a₁ + 18r = 15 13r = 13 /:13 r =1 a₁ = 2 - 5r a₁ = 2 - 5 a₁ = - 3 r = 1 b) an = a₁ + (n - 1)r an = - 3 + (n - 1)1 an = - 3 + n - 1 an = n - 4 c) S = 0,5(a₁₂ + a₄₇)35 a₁₂ = a₁ + 11r a₁₂ = -3 + 11 a₁₂ = 8 a₄₇ = a₁ + 46r a₄₇ = - 3 + 46 a₄₇ = 43 S = 0,5(8 + 43)35 S = 0,5*51*35 S = 892,5 d) S = 15 n = ? S = 0,5(a₁ + an)n S = 0,5[a₁ + a₁ + (n - 1)r]n S = 0,5[2a₁ + (n - 1)r]n 0,5(-6 + n - 1)n = 15 /*2 (n - 7)n = 30 n² - 7n - 30 = 0 Δ = 49 +120 = 169 √Δ = 13 n = (7 - 13)/2 lub n = (7 + 13)/2 n = - 3 lub n = 10 n ∉ N n = 10
