How many terms of the AP -17,-15,-13,...........are needed to make the sum -72?
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Step-by-step explanation:
let this is the sum of n terms
d=a2 -a1 = -15 - (-17)= -15+17 =2
Sn = n (2a+(n-1)d)/2
-72 = n (-34+ ( n-1)2)/2
-144 =-34n +2n^2- 2n
-72 =n^2-18n
n^2 -18n+72=0
(n-12) (n-6)=0
n=12 or 6
double answer means that sum of 7th term to 12th term is 0.
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