find the length of terms of the AP whose sum of first n term is 3n
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Answer:
Ln = 7 -3n ------(1)
l1 = 7- 3*1 = 7-3 =4
l2 = 7 - 3*2 = 7 - 6 =1
l3 = 7 - 3*3 = 7 - 9 = -2
first term = a = 4
common difference = d = l2 - l1 = 1-4 = -3
sum of 25 terms = sn = n/2[2a+(n-1)d]
s25 = 25/2 * [ 2*4 + (25-1)(-3)]
= 25/2 * [8 + 24(-3)]
= 25/2 * [ 8- 72]
= 25/2 * [-64]
= - 25 *32
= -800
therefore
sum of 25 terms in an ap is -800
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