How many terms of the AP :24, 21, 18,..
. must be taken so that their sum is 78?
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Answer:
Step-by-step explanation:
ANSWER
Formula,
Sn = 2/n [2a+(n−1)d]
Given,
a=24,d=21−24=−3,S n =78
78= n/2 [2(24)+(n−1)(−3)]
156=n[48−3n+3]
3n 2 −51n+156=0
n2−17n+52=0
(n−13)(n−4)=0
∴n=4,13
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