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Choose the correct alternative answer:
The first term of an A.P. is `1` and `nth` term is `20`. If `S_n=399`, then `n=?`

A

(a) 42

B

(d) 38

C

(c) 21

D

(d) 19

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NAVNEET PUBLICATION-ARITHMETIC PROGRESSION-EXERCISE
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  2. Solve :1+5+9+….+x=1770

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  13. Choose the correct alternative: For an A.P., t1=2, tn=41 and Sn=860....

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  18. Choose the correct alternative: The first term of an A.P. is 1 and n...

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  19. Find the first terem of the sequence: tn=3n+1

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  20. Find the first terem of the sequence: tn1/n^2-1

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  21. Find the first terem of the sequence: tn=4n-3

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