This is an arithmetic progression based question from Chapter name- Arithmetic Progression
Chapter number- 9
Exercise – 9.6
In this question we have been given the arithmetic progression whose nth term is given by an = 7 – 3n. we have to find the sum of it of first 25 terms.
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Understanding CBSE Mathematics
Class :- 10th
Question no 8
Given A.P. has nth term, an = 7 – 3n
and number of terms(n) = 25
So, an = 7 – 3(25) = -68
On putting n = 1, we get a, first term of the A.P.,
a = 7 – 3(1) = 4
S25 = n[a + an]/2
= 25(4 – 68)/2
= 25 x (-32)
= -800
Hence, the sum of the first 25 terms of given A.P. is -800.