Question

Question 6
The ${21}^{st}$ term of an AP whose first two terms are – 3 and 4, is

A) 17
B) 137
C) 143
D) -143

Answer 1

Given, the first two terms of the AP are :
a = -3 and
a + d = 4.
$\Rightarrow -3+d=4$
$\Rightarrow d=7$
$\therefore a_{21}=a+(21-1)d[\because a_n=a+(n-1\left)d\right]$
$\Rightarrow a_{21}=-3+\left(20\right)7$
$\Rightarrow a_{21}=-3+140=137$