if the sum of the first 6 terms of an AP is 9 and the sum of its first 9 terms is 6 , then find the sum of its first 15 terms
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sum of 6 terms=9
a+5d=9
sum of 9 terms=6
a+8d=6
now,
solving the two equation
subtract 2 from 1
a+5d=9
a+8d=6
- -. -
sign changes
a gets cancelled
equation comes to be
-3d=3
d=-1
putting d value in ist equation
a+5d=6
a-5=6
a=11
sum of 15 terms =a+14d
=a+14(-1)
=11-14
=-3
a+5d=9
sum of 9 terms=6
a+8d=6
now,
solving the two equation
subtract 2 from 1
a+5d=9
a+8d=6
- -. -
sign changes
a gets cancelled
equation comes to be
-3d=3
d=-1
putting d value in ist equation
a+5d=6
a-5=6
a=11
sum of 15 terms =a+14d
=a+14(-1)
=11-14
=-3
HappyntOtaku:
it's a wrong answer tho as sum of terms formula can't be a+ nd
can you just check the answer if you have it
kindly give right answer
sum can be a+ nd
the answer is -15 and the book says sum of the ap' formula as n/2 (2a + (n-1) d)
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