Which term of the AP : 3, 8, 13, 18, . . . , is 78?
___ $^{th}$

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Solution

Here, First term, $a = 3$

Common difference, $d = 8 - 3 = 5$
$a_{n} = 78$
Using formula $a_{n} = a + (n - 1) d$ to find nth term of arithmetic progression, we get
$a_{n} = 3 + (n - 1) 5$ {we want to find value of n here.}
$\Rightarrow 78 = 3 + (n - 1) 5$
$\Rightarrow 75 = 5 n - 5$
$\Rightarrow 80 = 5 n$
$\Rightarrow n = \frac{80}{5} = 16$
It means $16^{t h}$ term of the given AP is equal to 78.

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