Find how many terms of series 17+15+13+.... must be added to get sum equal to 72.
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Sum =‘17
Series is 17,15,13,....
a=17,d=−2
sn=2n[2(17)+(n−1)−2]72=2n(34−2n+2)144=36n−2n2n2−18n+72=0n2−12n−6n+72=0n(n−12)−6(n−12)=0n=6,12a6=17+5(−2)=7a12=17+11(−2)=−5
Since it is decreasing AP
The sum gets increases til the positive terms are encountered and decreases since negitive values are added.
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