\begin{tabular}{l}
4. In a geometric sequence, [tex]$S_n = p \left(1 - \left(\frac{1}{2}\right)^n\right)$[/tex] and [tex]$S_x = 10$[/tex]. Find the value of [tex]$p$[/tex]. \\
(a) Find the value(s) of [tex]$x$[/tex] for which [tex]$S_{\infty}$[/tex] exists. \\
(b) Choose any value for [tex]$x$[/tex] in this interval and calculate [tex]$S_{\infty}$[/tex]. \\
\hline
5. Show that the sequence [tex]$1, x + 1, x - 3, \ldots$[/tex] is not a geometric sequence. \\
\hline
6. The sum of the second and the third terms of a G.P. is 280, and the sum of the 5th and the 6th terms is 4375. Find the 4th term. \\
\hline
7. The first three terms of a geometric sequence are [tex]$4, -2, 1$[/tex]. Which term in this sequence will have a value of [tex]$\frac{1}{64}$[/tex]? \\
\hline
8. Twenty water tanks are decreasing in size in such a way that the volume of each tank is half the volume of the previous tank. The first tank is empty, but the other tanks are full to capacity. Will the first tank be able to hold all the water of the other 19 tanks? \\
\end{tabular}

Number Patterns: Mixed

9. The following sequence is given: [tex]$3, 3, 9, 6, 15, 12, \ldots$[/tex].
(a) Calculate [tex]$I_{52} - I_{51}$[/tex]. \\
(b) Will this sequence always be divisible by 3? Motivate your answer. \\
(c) Calculate the sum of the first 50 terms of this sequence.



Answer :

Let's solve the given questions one by one with detailed steps.

### Question 9: Sequence 3, 3, 9, 6, 15, 12, ...

#### (a) Calculate [tex]\( I_{52} - I_{51} \)[/tex]

1. The sequence alternates between two sub-sequences:
- Odd-indexed terms: [tex]\(3, 9, 15, \dots\)[/tex]
- Even-indexed terms: [tex]\(3, 6, 12, \dots\)[/tex]

2. The odd-indexed terms form an arithmetic sequence with the first term 3 and common difference of 6:
- General formula for odd-indexed terms: [tex]\( I_{2m-1} = 3 + 6 \times (m-1) \)[/tex]

3. The even-indexed terms also form an arithmetic sequence with the first term 3 and common difference of 6:
- General formula for even-indexed terms: [tex]\( I_{2m} = 3 + 6 \times (m-1) \)[/tex]

4. Determine the indices:
- [tex]\( I_{51} \)[/tex] is odd-indexed, so use the formula for odd-indexed terms with [tex]\( m = 26 \)[/tex].
- [tex]\( I_{52} \)[/tex] is even-indexed, so use the formula for even-indexed terms with [tex]\( m = 26 \)[/tex].

5. Calculate:
- [tex]\( I_{51} = 3 + 6 \times (26-1) = 3 + 6 \times 25 = 3 + 150 = 153 \)[/tex]
- [tex]\( I_{52} = 3 + 6 \times (26-1) = 3 + 6 \times 25 = 3 + 150 = 153 \)[/tex]

6. Therefore, [tex]\( I_{52} - I_{51} = 153 - 153 = 0 \)[/tex].

#### (b) Will this sequence always be divisible by 3? Motivate.

1. Both subsequences are arithmetic sequences with a common difference of 6.

2. Since the common difference, 6, is a multiple of 3, and both sequences start with 3 (which is divisible by 3), every term in both sequences will also be divisible by 3.

3. Therefore, the entire sequence will always be divisible by 3.

#### (c) Calculate the sum of the first 50 terms of this sequence.

1. The first 50 terms are comprised of 25 odd-indexed terms and 25 even-indexed terms.

2. For the odd-indexed subsequence ([tex]\(a = 3\)[/tex], [tex]\(d = 6\)[/tex], [tex]\(n = 25\)[/tex]):
- The sum of the first 25 odd-indexed terms, [tex]\( S_{odd} \)[/tex], can be calculated using the sum formula for an arithmetic sequence:
[tex]\[ S_{odd} = \frac{n}{2} (2a + (n-1)d) = \frac{25}{2} (2 \times 3 + (25-1) \times 6) = \frac{25}{2} (6 + 144) = \frac{25}{2} \times 150 = 1875 \][/tex]

3. For the even-indexed subsequence ([tex]\(a = 3\)[/tex], [tex]\(d = 6\)[/tex], [tex]\(n = 25\)[/tex]):
- The sum of the first 25 even-indexed terms, [tex]\( S_{even} \)[/tex], can be calculated similarly:
[tex]\[ S_{even} = \frac{n}{2} (2a + (n-1)d) = \frac{25}{2} (2 \times 3 + (25-1) \times 6) = \frac{25}{2} (6 + 144) = \frac{25}{2} \times 150 = 1875 \][/tex]

4. Total sum of the first 50 terms, [tex]\( S_{total} \)[/tex], is the sum of [tex]\( S_{odd} \)[/tex] and [tex]\( S_{even} \)[/tex]:
[tex]\[ S_{total} = S_{odd} + S_{even} = 1875 + 1875 = 3750 \][/tex]

### Summary of results:

1. [tex]\( I_{52} - I_{51} = \boxed{0} \)[/tex]
2. The sequence will always be divisible by 3.
3. The sum of the first 50 terms of the sequence is [tex]\( \boxed{3750} \)[/tex].