Question
Find:Which term in the A.P. $21,42,63,84, \ldots .$. is 420 ?
✓
Answer
In the given problem, we are given an $A.P$. and the value of one of its term. We need to find which term it is ( n ). So here we will find the value of $n$ using the formula, $a_n=a+(n-1) d$.
A.P. $21,42,63,84, \ldots . .420$
Here first term $( a )=21$
Common difference $( d )=42-21=21$
Now $a_n=a+(n-1) d$
$\Rightarrow 420=21+(n-1) \times 21$
$\Rightarrow 420=21+21 n-21 \Rightarrow 420=21 n$
$\Rightarrow n =\frac{420}{21}=20$
$\therefore 420$ is the $20^{\text {th }}$ term.
A.P. $21,42,63,84, \ldots . .420$
Here first term $( a )=21$
Common difference $( d )=42-21=21$
Now $a_n=a+(n-1) d$
$\Rightarrow 420=21+(n-1) \times 21$
$\Rightarrow 420=21+21 n-21 \Rightarrow 420=21 n$
$\Rightarrow n =\frac{420}{21}=20$
$\therefore 420$ is the $20^{\text {th }}$ term.
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