How many terms of the AP : 24, 21,18,... must be taken so that their sum is 78?

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Solution: a=24 and d=21-24=-3

S_{n} = \frac{n}{2} (2 a + (n - 1) d)

78 = \frac{n}{2} (2 (24) + (n - 1) (- 3))

156=n(48-3n+3)

n^{2} - 51 n + 156 = 0

n(n-13)-4(n-13)=0
(n-4)(n-13)
n=4,13
n can not be 13
so, n=3

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Question Text	How many terms of the AP : 24, 21,18,... must be taken so that their sum is 78?
Answer Type	Text solution:1
Upvotes	150