Choose the correct answer in each of the following questions:
The 7th term of an AP is −1 and its 16th term is 17. The nth term of the AP is
(a) $(3 n+8)$
(b) $(4 n-7)$
(c) $(15-2 n)$
(d) $(2 n-15)$
Let a be the first term and d be the common difference of the AP. Then,
nth term of the AP, an = a + (n − 1)d
Now,
$a_{7}=-1$ (Given)
$\Rightarrow a+6 d=-1 \quad \ldots(1)$
Also,
$a_{16}=17$ (Given)
$\Rightarrow a+15 d=17 \quad \ldots(2)$
Subtracting (1) from (2), we get
$(a+15 d)-(a+6 d)=17-(-1)$
$\Rightarrow 9 d=18$
$\Rightarrow d=2$
Putting d = 2 in (1), we get
$a+6 \times 2=-1$
$\Rightarrow a=-1-12=-13$
$\therefore n$th term of the AP, $a_{n}=-13+(n-1) \times 2=2 n-15$
Hence, the correct answer is option D.