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2) a3+a8+a10+a16+a18+a23=126,найдите сумму первых 25 членов an
a3 = a1 + 2d, a8 = a1 + 7d, a10 = a1 + 9d, a16 = a1 + 15d, a18 = a1 + 17d, a23 = a1 + 22d
a1*6 + 22d + 17d + 15d + 9d + 7d + 2d = a1*6 + 40d + 17d + 15d = a1*6 + 55d + 17d = a1*6 + 72d = 126
a1 + 12d = 21, a13 = 21
a1 = a13 - 12d
a2 = a13 - 11d
a3 = a13 - 10d
...
a12 = a13 - 1d
...
a14 = a13 +1d
...
a23 = a13 +10d
a24 = a13 + 11d
a25 = a13 + 12d
S_25 = a1 + a2 + a3 + ... + a23 + a24 + a25 = 25*a13 -12d -11d -10d - ... -d + d + ... + 10d + 11d + 12d = 25*a13 = 25*21 = 525
3m-5m=1/3-5/4
-2m=-11/12
m=11/24
<span>m(t) = m0·2</span><span>^(−t/T)</span>
<span>m0=200 <span>мг
</span></span><span>T=3 мин
</span>m(t) = <span>25 <span>мг
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t - ?
Подставляем в формулу </span></span><span>m(t) = m0·2<span>^(−t/T) </span>значения </span>m0, Т и m(t) :
<span>25 = 200·2</span><span><span>^(</span><span>− t/3)
</span></span>25 = 200/2<span>^(t/3)</span>
2<span>^(<span>t/3) = 200/25
</span></span>2<span>^(t/3) = 8</span>
2<span>^(<span>t/3) = 2³
</span></span><span>t/3 = 3
</span><span>t = 9
Ответ: 9 минут
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