the sum of the first n terms of an arithmetic progression is given by n square +5n find the 25th term of the arithmetic progression
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Step-by-step explanation:
When n=1 , S1=a1=6(1)-1^2= 5. …………..[1]
When n=2 , S2=a1+a2=6(2)-2^2=8……...[2]
As we know , a1 =S1=5. …………………….[3]
And S2=a1+ a2.(now put a1=5 as per [3])
We get, 8 =5+a2 => a2=3..………………….[4]
Now, d=a2-a1=3-5=-2.
Formula for a^n term of an A.p. is
=a+(n-1)d
Therefore,
25th term is as follows, ( where n=25)
a25=a+(n-1)d
a25=5+(25-1)(-2)
a25=5+(24)(-2)=-43.
So, 25th term is -43.
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