The sum of the first 10th terms of an arithmetic series is 50 and 5th term is three times of the second term.calculate the sum of the first 15th term of the series
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SUM of first ten terms = 10/2 (2a +9d) =50.
2a +9d = 10. .. ..(1)
5 th term = 3 × 2nd term
a + 4d = 3(a +d)
a + 4d = 3a +3d
2a -- d = 0
a = d/2 .. . ..(2)
By (1) & (2)
d =1 a = 1/2
Sum of 20 terms = 20/2 (1 + 19)
=200
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