Which term of the A.P
$3, 8, 13, 18, \dots$ is 78?

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Solution

$G i v e n t h e A . P i s 3, 8, 13, 18, \dots$
Here,
$a = 3 d = a_{2} - a_{1} = 8 - 3 = 5 L e t n^{t h} t e r m o f t h i s A . P . b e 78.$

$T h e n^{t h} t e r m a n A . P . i s g i v e n b y :$
$a_{n} = a + (n - 1) d \Rightarrow 78 = 3 + (n - 1) 5 75 = (n - 1) 5 (n - 1) = 15 n = 16 H e n c e, t h e 16^{t h} t e r m o f t h i s A . P . i s 78.$

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