Question

If the $9^{\text{th}}$ term of an AP is 15 and the common difference is 2, then the $1^{\text{st}}$ term of the AP is ___.

• A. 1
• B. -1
• C. 2

Answer 1

The correct option is B -1

The $n^{\text{th}}$ term of an AP with first term $a$ and common difference $d$ is given by
$a_n=a+(n-1)d.$
Given that $a_9=15$ and $d=2.$
$\Rightarrow a+(9-1)2=15$
$\Rightarrow a+16=15$
$\Rightarrow a=-1$
Thus, the first term of the AP is -1.