一个有限数列 {an}\{a_n\} 含有 50 项,且对于 n=1,2,3,…,49n = 1,2,3,\ldots,49,满足 an+1=an+0.15a_{n+1} = a_n + 0.15。 另一个无限数列 {bm}\{b_m\} 满足 b1=a50b_1 = a_{50} 且对于 m≥2m \ge 2,有 bmbm−1=0.98\dfrac{b_m}{b_{m-1}} = 0.98。

  1. (i)

    已知 a50=99a1a_{50} = 99 a_1,证明 a1=0.075a_1 = 0.075。

  2. (ii)

    在不使用计算器的情况下,求 ∑n=150an\sum_{n=1}^{50} a_n 的值。

  3. (iii)

    确定满足 bk<a25b_k < a_{25} 的最小 kk 值。

  4. (iv)

    求出最小的 hh 值,使得数列 {bm}\{b_m\} 前 hh 项的和大于其无穷级数和的 99%。

  5. (v)

    若改为 bmbm−1=−0.98\dfrac{b_m}{b_{m-1}} = -0.98,求 ∑m=0∞b1+3m\sum_{m=0}^{\infty} b_{1+3m} 的值。

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完整解法

4(i)

1. Identify the sequence as an arithmetic progression and express $a_{50}$ in terms of $a_1$.

Since an+1−an=0.15a_{n+1} - a_n = 0.15 is a constant, {an}\{a_n\} is an arithmetic sequence with a common difference d=0.15d = 0.15.
The 50th term is given by the formula an=a1+(n−1)da_n = a_1 + (n-1)d:
a50=a1+(50−1)(0.15)a_{50} = a_1 + (50 - 1)(0.15)
a50=a1+49(0.15)a_{50} = a_1 + 49(0.15)

2. Equate to the given relation and solve for $a_1$.

Substitute a50a_{50} into the given equation a50=99a1a_{50} = 99a_1:
a1+49(0.15)=99a1a_1 + 49(0.15) = 99a_1
98a1=49(0.15)98a_1 = 49(0.15)
Divide both sides by 4949:
2a1=0.152a_1 = 0.15
a1=0.075 (shown)a_1 = 0.075 \text{ (shown)}

答案

a1=0.075a_1 = 0.075 (shown)

4(ii)

1. Apply the sum formula for an arithmetic progression.

The sequence is an arithmetic progression with n=50n = 50. Using the sum formula Sn=n2(a1+an)S_n = \frac{n}{2}(a_1 + a_n):
∑n=150an=502(a1+a50)\sum_{n=1}^{50} a_n = \frac{50}{2} (a_1 + a_{50})

2. Substitute the relation $a_{50} = 99a_1$ and $a_1 = 0.075$ to evaluate the sum without a calculator.

Substitute a50=99a1a_{50} = 99a_1 (given in part (i)):
∑n=150an=25(a1+99a1)\sum_{n=1}^{50} a_n = 25(a_1 + 99a_1)
=25(100a1)= 25(100a_1)
Substitute a1=0.075a_1 = 0.075:
=25(100×0.075)= 25(100 \times 0.075)
=25×7.5=187.5= 25 \times 7.5 = 187.5

(Alternatively, using Sn=n2[2a1+(n−1)d]S_n = \frac{n}{2}[2a_1 + (n-1)d]:
S50=25[2(0.075)+49(0.15)]=25[0.15+49(0.15)]=25[50(0.15)]=25(7.5)=187.5S_{50} = 25[2(0.075) + 49(0.15)] = 25[0.15 + 49(0.15)] = 25[50(0.15)] = 25(7.5) = 187.5)

答案

187.5187.5

4(iii)

1. Calculate the values of $a_{25}$ and $b_1$.

a25=a1+24da_{25} = a_1 + 24d
a25=0.075+24(0.15)=3.675a_{25} = 0.075 + 24(0.15) = 3.675

b1=a50=99a1b_1 = a_{50} = 99a_1
b1=99(0.075)=7.425b_1 = 99(0.075) = 7.425

2. Formulate the inequality $b_k < a_{25}$ using the $n$th term formula for a geometric sequence.

bk<a25b_k < a_{25}
b1(r)k−1<a25b_1(r)^{k-1} < a_{25}
7.425(0.98)k−1<3.6757.425(0.98)^{k-1} < 3.675
(0.98)k−1<3.6757.425(0.98)^{k-1} < \frac{3.675}{7.425}
(0.98)k−1<4999(0.98)^{k-1} < \frac{49}{99}

3. Solve for the smallest integer $k$ using logarithms.

Taking the natural logarithm on both sides:
(k−1)ln⁡(0.98)<ln⁡(4999)(k-1) \ln(0.98) < \ln\left(\frac{49}{99}\right)

Since ln⁡(0.98)\ln(0.98) is negative, dividing by it reverses the inequality sign:
k−1>ln⁡(49/99)ln⁡(0.98)k - 1 > \frac{\ln(49/99)}{\ln(0.98)}
k−1>34.811k - 1 > 34.811
k>35.811k > 35.811

The smallest integer value of kk is 3636.

答案

3636

4(iv)

1. Set up an inequality comparing the sum of the first $h$ terms ($S_h$) to the sum to infinity ($S_\infty$).

Sh>0.99S∞S_h > 0.99 S_\infty
b1(1−0.98h)1−0.98>0.99(b11−0.98)\frac{b_1(1 - 0.98^h)}{1 - 0.98} > 0.99 \left( \frac{b_1}{1 - 0.98} \right)

2. Simplify the inequality by cancelling common factors.

Since the first term b1=a50=0.075+49(0.15)=7.425b_1 = a_{50} = 0.075 + 49(0.15) = 7.425 is positive, we can divide both sides by b11−0.98\frac{b_1}{1 - 0.98} without reversing the inequality sign:
1−0.98h>0.991 - 0.98^h > 0.99
0.98h<0.010.98^h < 0.01

3. Solve the exponential inequality using logarithms.

Taking the natural logarithm on both sides:
ln⁡(0.98h)<ln⁡(0.01)\ln(0.98^h) < \ln(0.01)
hln⁡(0.98)<ln⁡(0.01)h \ln(0.98) < \ln(0.01)
Since ln⁡(0.98)\ln(0.98) is negative, dividing by it reverses the inequality sign:
h>ln⁡(0.01)ln⁡(0.98)h > \frac{\ln(0.01)}{\ln(0.98)}
h>227.95h > 227.95

4. State the least integer value for $h$.

Since hh represents the number of terms, it must be an integer.
The least value of h=228h = 228.

答案

228228

4(v)

1. Determine the first term and common ratio of the new series

From earlier parts, a1=0.075a_1 = 0.075 and the common difference for ana_n is 0.150.15.
b1=a50=a1+49d=0.075+49(0.15)=7.425b_1 = a_{50} = a_1 + 49d = 0.075 + 49(0.15) = 7.425

The series to evaluate is ∑m=0∞b1+3m=b1+b4+b7+…\sum_{m=0}^{\infty} b_{1+3m} = b_1 + b_4 + b_7 + \dots
This is a new geometric series with the first term A=b1=7.425A = b_1 = 7.425.
Since the consecutive ratio of bmb_m is r=−0.98r = -0.98, the common ratio for this new series is taking jumps of 3 terms:
R=r3=(−0.98)3=−0.941192R = r^3 = (-0.98)^3 = -0.941192

2. Calculate the sum to infinity of the new series

S∞=A1−R=7.4251−(−0.98)3=7.4251+0.941192=7.4251.941192≈3.82497\begin{aligned} S_\infty &= \frac{A}{1 - R} \\ &= \frac{7.425}{1 - (-0.98)^3} \\ &= \frac{7.425}{1 + 0.941192} \\ &= \frac{7.425}{1.941192} \\ &\approx 3.82497 \end{aligned}

∴∑m=0∞b1+3m=3.82 (to 3 s.f.)\therefore \sum_{m=0}^{\infty} b_{1+3m} = 3.82 \text{ (to 3 s.f.)}

答案

3.823.82

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