Răspuns:
Explicație pas cu pas:
3) progresie aritmetica : 2 ; 3x²-2x-1 ; 12 ; 17 ....=>
r = 17-12 = 5 => 12-5 = 7 (termenul lipsa) ; 7-5 = 2 =>
3x²-2x-1 = 7 => 3x²-2x-8 = 0 => x₁,₂ = [2±√(4+96)]/6 =>
x₁,₂ = (2±10)/6 , => x₁ = -4/3 ; x₂ = 2
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4){a₅+a₉ = 26 <=> {a₁+4r+a₁+8r = 26
{a₁-r = 19/2 {2a₁ -2r = 19 =>
{2a₁+12r = 26
{-2a₁+2r = -19
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/ 14r = 26-19 = 7 => r = 7/14 => r = 1/2
a₁ = r +19/2 = 1/2 + 19/2 = 20/2 => a₁ = 10
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5) a₂ = 10 ; a₇ = 18 si 1/2 = 37/2
a₇ = a₂+5r => 5r = a₇-a₂ = 37/2 - ²⁾10 = 17/2 =>
r = 17/2 : 5 => r = 17/10
#copaceibrainly
