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### Arithmetic Series
The general formula for the nth term \( a_n \) and sum \( S_n \) of an arithmetic series are:
- \( a_n = a_1 + (n-1)d \)
- \( S_n = \frac{n}{2} \cdot (a_1 + a_n) \)

Where:
- \( a_1 \) is the first term
- \( d \) is the common difference
- \( n \) is the number of terms

#### 1. \( 7, 11, 15, \dots \)
- **Common difference** \( d = 11 - 7 = 4 \)
- Find \( a_7 \):  
  \( a_7 = 7 + (7-1) \times 4 = 7 + 24 = 31 \)
  
- Find \( S_7 \):  
  \( S_7 = \frac{7}{2} \times (7 + 31) = \frac{7}{2} \times 38 = 133 \)

#### 2. \( 18, 11, 4, \dots \)
- **Common difference** \( d = 11 - 18 = -7 \)
- Find \( a_6 \):  
  \( a_6 = 18 + (6-1) \times (-7) = 18 - 35 = -17 \)
  
- Find \( S_6 \):  
  \( S_6 = \frac{6}{2} \times (18 + (-17)) = 3 \times 1 = 3 \)

#### 3. \( -7, -5, -3, \dots \)
- **Common difference** \( d = -5 - (-7) = 2 \)
- Find \( a_{23} \):  
  \( a_{23} = -7 + (23-1) \times 2 = -7 + 44 = 37 \)
  
- Find \( S_{23} \):  
  \( S_{23} = \frac{23}{2} \times (-7 + 37) = \frac{23}{2} \times 30 = 345 \)

#### 4. \( 3, 3 \frac{1}{2}, 4, \dots \)
- **Common difference** \( d = 3.5 - 3 = 0.5 \)
- Find \( a_{16} \):  
  \( a_{16} = 3 + (16-1) \times 0.5 = 3 + 7.5 = 10.5 \)
  
- Find \( S_{16} \):  
  \( S_{16} = \frac{16}{2} \times (3 + 10.5) = 8 \times 13.5 = 108 \)

#### 5. \( -17, -12, -7, \dots \)
- **Common difference** \( d = -12 - (-17) = 5 \)
- Find \( a_{24} \):  
  \( a_{24} = -17 + (24-1) \times 5 = -17 + 115 = 98 \)
  
- Find \( S_{24} \):  
  \( S_{24} = \frac{24}{2} \times (-17 + 98) = 12 \times 81 = 972 \)

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### Geometric Series
The general formula for the nth term \( a_n \) and sum \( S_n \) of a geometric series are:
- \( a_n = a_1 \times r^{n-1} \)
- \( S_n = a_1 \times \frac{1 - r^n}{1 - r} \) (for \( |r| < 1 \))
- Sum of infinite series \( S_{\infty} = \frac{a_1}{1 - r} \) (for \( |r| < 1 \))

#### 1. \( -1, 6, -36, 216, \dots \)
- **Common ratio** \( r = \frac{6}{-1} = -6 \)
- Find \( a_{19} \):  
  \( a_{19} = -1 \times (-6)^{18} = -6^{18} \)
  \( a_{19} \) is a very large value.
  
- Find \( S_{19} \) using the sum formula for a geometric series (not suitable for this large common ratio).

#### 2. \( \frac{1}{2}, \frac{1}{8}, \frac{1}{32}, \dots \)
- **Common ratio** \( r = \frac{1/8}{1/2} = \frac{1}{4} \)
- Find \( a_{27} \):  
  \( a_{27} = \frac{1}{2} \times \left(\frac{1}{4}\right)^{26} \)
  
- Find \( S_{27} \) and infinite sum (since \( |r| < 1 \)) using geometric formulas.

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Would you like detailed calculations for the geometric series or any other part? 

Here are 5 related questions:
1. What is the nth term formula for a quadratic sequence?
2. Can the sum formula for an arithmetic series apply to negative sequences?
3. What happens if the common difference in an arithmetic sequence is fractional?
4. How do you determine the common ratio for fractional geometric sequences?
5. What is the application of infinite series sums in real-life scenarios?

**Tip:** In geometric series, ensure the common ratio's magnitude is less than 1 to use the infinite sum formula effectively.

Find the nth term and the sum of the following arithmetic series and geometric series.

Learn how to calculate the nth term and sum of arithmetic and geometric series using essential formulas. The problem covers multiple examples with step-by-step solutions, including how to find terms and sums for both finite and infinite series. Suitable for students in Grades 10-12.

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