Ответы на вопрос:
1. a19 = a1+18d = 30+18*(-2) = 30-36 = -6 2. d = a2-a1 = -) = -10+16 = 6 s(17) = (2*a1+16d)/2*17 = (-32+96)/2*17 = 544 3. a1 = 2+5 = 7 a2 = 2+10 = 12 a3 = 2+15 = 17 a2-a1 = 12-7 = 5 a3-a2 = 17-12 = 5 a2-a1 = a3-a2 4. a7 = a1+6d d = (a7-a1)/6 = (-9-3)/6 = -2 an = a1+(n-1)d -35 = 3+(n-1)*(-2) -35 = 3-2n+2 -35 = 5-2n 2n = 40 n = 20 a20 = a1+19d = 3+19*(-2) = 3-38 = -35 5. a1 = 1, d = 2. s(50) = (2*a1+49d)/2*50 = (2*1+49*2)*25 = (2+98)*25 = 100*25 = 2500
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