how many terms of the series -8,-6,-4 make the sum 90? options a17 b16 c15 d14
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Answer:
apply sum of an AP formula and find it
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Step-by-step explanation:
the series is -8,-6,-4
common difference ,d =2(-6-(-8)=2)
formula
sn=,n/2+(2a+(n-1)d)
90 = n/2+(2(-8)+(n-1)2)
180=n(-16+2n-2)
180=n(2n-18)
180=2n^2-18n
2n^2-18n-180=0
÷ by 2
n^2-9n-90=0
(n-15)(n+6)=0
n=15,n=-6
- n=15
- c is correct answer
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