х3 + 2х2 – 5х – 6 = 0
a1=2*1+3=5
a5=2*5+3=13
a2=2*2+3=7
d=7-5=2
a) S5=(a1+a5)*n/2=(5+13)*5/2=18*5/2=9*5=45
a13= 2*13-3=23
б) S13=(5+23)*13/2=28*13/2=14*13=182
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