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Question
Which term of the A.P. 3, 8, 13, 18, … is 78?
Solution
3, 8, 13, 18, … is 78
Let 78 is nth term
Here, a = 3, d = 8 – 3 = 5
∴ 78 = a + (n –1)d
⇒ 78 = 3 + (n – 1)5
⇒ 78 = 3 + 5n – 5
⇒ 78 + 5 – 3 = 5n
⇒ 5n = 80
⇒ n = `(80)/(5)` = 16
∴ 78 is 16th term.
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