Question:
In an AP, if d = – 4, n = 7 and an = 4, then a is equal to
(a) 6
(b) 7
(c) 20
(d) 28
Solution:
(d) In an AP $a_{n}=a+(n-1) d$
$\Rightarrow \quad 4=a+(7-1)(-4) \quad$ [by given conditions]
$\Rightarrow \quad 4=a+6(-4)$
$\Rightarrow \quad 4+24=a$
$\therefore \quad a=28$