Class 11 Mathematics Sequence and Series: NCERT Solutions for Question 13
If the sum of n terms of an A.P. is 3n2 + 5n and its mth term is 164, find the value of m.
Let a and b be the first term and the common difference of the A.P. respectively.
am = a + (m – 1)d = 164 … (1)
Sum of n terms,
Here,
Comparing the coefficient of n2 on both sides, we obtain
Comparing the coefficient of n on both sides, we obtain
Therefore, from (1), we obtain
8 + (m – 1) 6 = 164
⇒ (m – 1) 6 = 164 – 8 = 156
⇒ m – 1 = 26
⇒ m = 27
Thus, the value of m is 27.
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1 Comment(s) on this Question
Fabulous effort
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