3. Given an arithmetic sequence \{a_n\} with a_5 = 15, a_{20} = 45, find (i) a_n (ii) \sum_{k=1}^{100} a_k (iii) \sum_{k=60}^{150} a_k
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Using the given information: a5 = 15 a1 + (5-1)d = 15 a1 + 4d = 15 a20 = 45 a1 + (20-1)d = 45 a1 + 19d = 45 Now we have a system of equations: a1 + 4d = 15 a1 + 19d = 45 Show more…
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