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Mathematics

Which term of the AP : 3, 8, 13, 18, ……., is 78 ?

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Answer

In above A.P.,

3, 8, 13, 18, …….

First term (a) = 3

Common difference (d) = 8 - 3 = 5.

Let nth term of the A.P. be 78.

By formula,

⇒ an = a + (n - 1)d

Substituting values we get :

⇒ 78 = 3 + (n - 1) × 5

⇒ 78 = 3 + 5n - 5

⇒ 78 = 5n - 2

⇒ 5n = 78 + 2

⇒ 5n = 80

⇒ n = 805\dfrac{80}{5}

⇒ n = 16.

Hence, 16th term of the A.P. is 78.

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