Question

Which term of the A.P. $21,42,63,84,\dots \dots ..$ is $210.$

• A. $9^{th}$
• B. ${10}^{th}$
• C. ${11}^{th}$
• D. ${12}^{th}$

Answer 1

The correct option is B ${10}^{th}$

$21,42,63,\mathrm{84......}is210$.

Let nth term be $210$

$a=21$ $d=21$

$t_n=a+(n-1)d$

$210=21+(n-1)21$

$\frac{210}{21}=1+n-1$

$n=10$

n$=10th$ term