Question

The 21^st term of the A.P. whose first two terms are – 3 and 4 is ____________.

Answer 1

Given:
First term (a) = –3
Second term (a₂) = 4
Common difference (d) = a₂ – a₁
= $4-\left(-3\right)$
= 4 + 3
= 7

The 21^st term is:
$$\begin{gathered} a_{21}=a+\left(21-1\right)d \\ =-3+20\left(7\right) \\ =-3+140 \\ =137 \end{gathered}$$

​Hence, the 21^st term of the A.P. whose first two terms are –3 and 4 is 137.