Question 92

The ﬁrst and last terms of an arithmetic progression are 29 and -49. If the sum of the series is -140, then it has how many terms?

Solution

In an arithmetic progression with first term, $$a = 29$$ , last term, $$l = -49$$

Let number of terms = $$n$$

$$\therefore$$ Sum of A.P. = $$\frac{n}{2} (a + l) = -140$$

=> $$\frac{n}{2} (29 - 49) = -140$$

=> $$\frac{-20n}{2} = -140$$

=> $$n = \frac{(-140) \times 2}{-20} = 7 \times 2$$

=> $$n=14$$

=> Ans - (B)

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