Q.1) The first and last term of an AP is 4 and 81 respectively. If the common difference is 7, how many terms are there in the AP and what is their sum?
Q.2) The sum of n terms of an AP is 5n^2-3n.Find the AP and also it's 10th term
Answers
Answered by
0
n-12 will be the right answer hope it helps u
RohitKumarSharma0000:
I need the full derivation!!!!!
Answered by
3
Hey...
ur answer...
1) l=a+(n-1)d
=81=4 +7n -7
=81-4+7=7n
= 7n=84
= n=12
S10 =n/2 (a+l)
= 12/2(81+4)
=6×85
= 450
2)Sn=5n2 -3n
S(n-1)= 5(n-1)2 -3 (n-1)
=5n2 +5-10n -3n+3
= 5n2 -13n +8
n=Sn-S (n-1)
n= 5n2 -3n -5n2 +13n -8
n=10n-8
now..
S10 =10×10-8
=100-8
=92
Hope this helps u....
plz mark as brainliest answer....
ur answer...
1) l=a+(n-1)d
=81=4 +7n -7
=81-4+7=7n
= 7n=84
= n=12
S10 =n/2 (a+l)
= 12/2(81+4)
=6×85
= 450
2)Sn=5n2 -3n
S(n-1)= 5(n-1)2 -3 (n-1)
=5n2 +5-10n -3n+3
= 5n2 -13n +8
n=Sn-S (n-1)
n= 5n2 -3n -5n2 +13n -8
n=10n-8
now..
S10 =10×10-8
=100-8
=92
Hope this helps u....
plz mark as brainliest answer....
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