1. **Problem statement:** Given the sequence 5, 11, 17, 23, 29, find:
(a) the expression for the nth term,
(b) the sum of the first 10 terms,
(c) which term equals 119.
2. **Identify the sequence type:** The sequence increases by 6 each time (11-5=6, 17-11=6), so it is an arithmetic sequence.
3. **Formula for nth term of an arithmetic sequence:**
$$a_n = a_1 + (n-1)d$$
where $a_1$ is the first term and $d$ is the common difference.
4. **Find nth term expression:**
Given $a_1=5$ and $d=6$,
$$a_n = 5 + (n-1)6 = 5 + 6n - 6 = 6n - 1$$
5. **Sum of first n terms formula:**
$$S_n = \frac{n}{2}(a_1 + a_n)$$
6. **Sum of first 10 terms:**
Calculate $a_{10}$:
$$a_{10} = 6(10) - 1 = 60 - 1 = 59$$
Then,
$$S_{10} = \frac{10}{2}(5 + 59) = 5 \times 64 = 320$$
7. **Find which term equals 119:**
Set $a_n = 119$:
$$6n - 1 = 119$$
Add 1 to both sides:
$$6n = 120$$
Divide both sides by 6:
$$n = \frac{\cancel{6}n}{\cancel{6}} = \frac{120}{6} = 20$$
**Final answers:**
(a) $a_n = 6n - 1$
(b) $S_{10} = 320$
(c) The 20th term is 119.
Arithmetic Sequence 353Bdc
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