How many terms of the ap -6,-11/2,-5 make the sum-25
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Answer:
5th or 20th
Step-by-step explanation:
Sum of n terms of AP = Sₙ = n/2[2a + (n-1)d]
a = -6, Sₙ = - 25, d = -11 /2+6 = 1/2
-25 = n/2[ - 12 + (n-1) 1/2]
- 25 = n/2[[-24 + n - 1 /2]
- 25= n/4[n - 25]
n² -25n + 100 = 0
n² -20n - 5n + 100 = 0
n(n - 20) - 5 (n-20) = 0
(n-5) (n-20) = 0
n = 5 or 20.
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