Which term of the a.p .5,9,13,...is 81? Also find the sum 5+9+13+......?81.
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first term,a=5 , common difference,d=4
Let 81 be the nth term
Tn=a+(n-1)d
81=5+(n-1)4
(n-1)4=76
n-1=19
n=20
Hence 81 is the 20th term
Sn=n/2{2a+(n-1)d}
Sum of the 20 terms = 20/2{2*5+19*4} = 10(10+76) = 860
Let 81 be the nth term
Tn=a+(n-1)d
81=5+(n-1)4
(n-1)4=76
n-1=19
n=20
Hence 81 is the 20th term
Sn=n/2{2a+(n-1)d}
Sum of the 20 terms = 20/2{2*5+19*4} = 10(10+76) = 860
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