Question
The 4th term of an AP is 17 and the 9th term is 42. Find
the sum of the first 20 terms.Solution
ATQ,
d = (42 – 17) / 5 = 5 a = 17 – 3×5 = 2 S20 = 10 × (2×2 + 19×5) = 10 × 99 = 990
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