Как найти n и аn в арифметической прогрессии,если d=0,2, a1=0,1, Sn =22,5

Sn = (a1 + an)/2 * n

an = a1 + d(n - 1)

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22.5 = (0.1 + an)/2 * n

45 = (0.1 + an) * n

an = 0.1 + 0.2(n - 1)

45 = (0.2 + 0.2(n - 1))*n

225 = (1 + n - 1)*n

n² = 225

n = -15 нет n > 0

n = 15

an = a(15) = 0.1 + 0.2(n - 1) = 0.1 + 0.2*14 = 2.9