а1=-8

а2=-6

а3=-4

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1)d=a(n+1)-an = -4 - (-6) = -4 + 6 = 2

2)an=a1+(n-1)d.

a10 = -8 + 9 * 2 = -8 + 18 = 10

3)Sn=n*(a1+an)/2.

a15 = -8+14*2=-8+28=20

S15 = 15*(-8+20)/2=(15*12)/2=180/2=90

ОТВЕТ: a10=10; S15=90.