一个有限数列 含有 50 项,且对于 ,满足 。 另一个无限数列 满足 且对于 ,有 。
- (i)
已知 ,证明 。
- (ii)
在不使用计算器的情况下,求 的值。
- (iii)
确定满足 的最小 值。
- (iv)
求出最小的 值,使得数列 前 项的和大于其无穷级数和的 99%。
- (v)
若改为 ,求 的值。
它不会直接告诉你答案,而是问你卡在哪一步看看 AI 是怎么陪你一步步弄懂这道题的
完整解法
4(i)
1. Identify the sequence as an arithmetic progression and express $a_{50}$ in terms of $a_1$.
Since is a constant, is an arithmetic sequence with a common difference .
The 50th term is given by the formula :
2. Equate to the given relation and solve for $a_1$.
Substitute into the given equation :
Divide both sides by :
(shown)
4(ii)
1. Apply the sum formula for an arithmetic progression.
The sequence is an arithmetic progression with . Using the sum formula :
2. Substitute the relation $a_{50} = 99a_1$ and $a_1 = 0.075$ to evaluate the sum without a calculator.
Substitute (given in part (i)):
Substitute :
(Alternatively, using :
)
4(iii)
1. Calculate the values of $a_{25}$ and $b_1$.
2. Formulate the inequality $b_k < a_{25}$ using the $n$th term formula for a geometric sequence.
3. Solve for the smallest integer $k$ using logarithms.
Taking the natural logarithm on both sides:
Since is negative, dividing by it reverses the inequality sign:
The smallest integer value of is .
4(iv)
1. Set up an inequality comparing the sum of the first $h$ terms ($S_h$) to the sum to infinity ($S_\infty$).
2. Simplify the inequality by cancelling common factors.
Since the first term is positive, we can divide both sides by without reversing the inequality sign:
3. Solve the exponential inequality using logarithms.
Taking the natural logarithm on both sides:
Since is negative, dividing by it reverses the inequality sign:
4. State the least integer value for $h$.
Since represents the number of terms, it must be an integer.
The least value of .
4(v)
1. Determine the first term and common ratio of the new series
From earlier parts, and the common difference for is .
The series to evaluate is
This is a new geometric series with the first term .
Since the consecutive ratio of is , the common ratio for this new series is taking jumps of 3 terms:
2. Calculate the sum to infinity of the new series
AI 分析,可能出错
用 AI 批改,圈出写错的那一步,并自动将你的薄弱点存入错题本,形成你的错因库。