What is the sum of the first ten terms of the AP if the first terms is 1and last term is 19
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Correct option is
B
38
The is given that,
First term (a)=1
Last term (t
n
)=20
Sum of terms (S
n
)=399
We know that,
t
n
=a+(n−1)d
S
n
=
2
n
(2a+(n−1)d)
S
n
=
2
n
(a+(a+(n−1)d))
⇒S
n
=
2
n
(a+t
n
)
⇒399=
2
n
(1+20)
⇒399=
2
21n
⇒21n=399×2
⇒n=
21
798
⇒n=38
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