A finite sequence has 50 terms and is such that for . Another infinite sequence is such that and for .
- (i)
Given that , show that .
- (ii)
Find, without using a calculator, the value of
- (iii)
Determine the smallest value of such that .
- (iv)
Find the least value of such that the sum of the first terms of is more than 99% of its sum to infinity.
- (v)
If instead, find
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Worked solution
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
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