Find the sum of first 25th term of an AP whose nth term is 1 - 4n
CLASS 10TH CBSE SAMPLE PAPER 2018-19
MATHEMATICS
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Here it is given that, an = 1 - 4n
putting, n =1 , a1 = 1- 4(1) = -3
putting, n =2 , a2 = 1 - 4(2) = -7
Therefore, a = -3 and d = (-4)
so, S25 = 25/2 [ 2(-3) + 24 (-4)]
= 25/2 [-102]
= 25 (-51 )
= -1275 Ans
hence, the sum of first 25 terms of an AP whose nth term is 1 -4n is -1275.
Here it is given that, an = 1 - 4n
putting, n =1 , a1 = 1- 4(1) = -3
putting, n =2 , a2 = 1 - 4(2) = -7
Therefore, a = -3 and d = (-4)
so, S25 = 25/2 [ 2(-3) + 24 (-4)]
= 25/2 [-102]
= 25 (-51 )
= -1275 Ans
hence, the sum of first 25 terms of an AP whose nth term is 1 -4n is -1275.
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