How many Terms of Ap. 24 21 18 taken so that must be
taken so that their sum is 78
rahul751046:
n can be 13 or 4
Answers
Answered by
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Step-by-step explanation:
a = 24
d = 21-24= -3
sn= 78
sn = n/2 (2a + ( n-1) d)
78 = n/2 (48 + (n-1) -3)
78×2 =n (48 + (n-1)-3)
156=n(48 -3n+3)
156 =n (51 -3n)
-3n²+51n-156 ×-1
3n²- 51n + 156 ÷by 3
n² - 17n + 52
n² - 13n - 4n + 52
n(n-13) -4 (n-13)
n-13= 0 | n-4 = 0
n = 13 | n= 4
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