5 times of 5th term is equal to 8 times of 8th term of an arithmetic sequence then find the sum of first 25 term
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Answer:
given , 5(a + 4d) = 8(a + 7d)
=> 5a + 20d = 8a + 56d
=> -3a - 36d = 0
=> a + 12d = 0 [ divide both sides with -3]
S25 = 25/2 [ 2a + 24d ]
=> 25/2*2 [ a + 12d] [ factor out 2 ]
=> 25 × 0
=> 0 answer
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