Question.
Which term of the AP : 3, 8, 13, 18, ... is 78?
Which term of the AP : 3, 8, 13, 18, ... is 78?
Solution:
$a=3, d=5$
Let $\mathrm{t}_{\mathrm{n}}=78$
$\Rightarrow a+(n-1) d=78$
$\Rightarrow 3+(n-1) \times 5=78 \Rightarrow 5 n-2=78$
$\Rightarrow 5 \mathrm{n}=80 \quad \Rightarrow \mathrm{n}=16$
Hence, $\mathrm{t}_{16}=78$
$a=3, d=5$
Let $\mathrm{t}_{\mathrm{n}}=78$
$\Rightarrow a+(n-1) d=78$
$\Rightarrow 3+(n-1) \times 5=78 \Rightarrow 5 n-2=78$
$\Rightarrow 5 \mathrm{n}=80 \quad \Rightarrow \mathrm{n}=16$
Hence, $\mathrm{t}_{16}=78$
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