Question:

What is the sum of the 40th and the 70th elements of the series defined as: \[ s_n = s_{n-1} - 5, \quad s_1 = 281 \]

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For a recurrence relation like \( s_n = s_{n-1} - 5 \), you can express it in terms of the general formula \( s_n = s_1 - 5(n-1) \) to calculate the nth term.
Updated On: Sep 30, 2025
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The Correct Option is A

Solution and Explanation

The general formula for the recurrence relation is:
\[ s_n = s_1 - 5(n-1) \] Step 1: Find \( s_{40} \):
\[ s_{40} = 281 - 5(40-1) = 281 - 5 \times 39 = 281 - 195 = 86 \]
Step 2: Find \( s_{70} \):
\[ s_{70} = 281 - 5(70-1) = 281 - 5 \times 69 = 281 - 345 = -64 \]
Step 3: Add \( s_{40} \) and \( s_{70} \):
\[ s_{40} + s_{70} = 86 + (-64) = 22 \]
Final Answer: \[ \boxed{22} \]
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