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  4. In an A.P., the first term is 3 and the last term is 17. The sum of all the terms in the sequence is 70. Then the number of terms in the arithmetic sequence is

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Q. In an A.P., the first term is $3$ and the last term is $17$. The sum of all the terms in the sequence is $70$. Then the number of terms in the arithmetic sequence is

KEAMKEAM 2020

Solution:

$\frac{n}{2}\left[t_{1}+t_{n}\right]=70$
$n[3+17]=140$
$n \times 20=140$
$n=7$