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Which term of the AP 5, 9, 13, 17, …….. is 81? |
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Answer» Given AP is 5, 9, 13, 17, ........ Hence, the first term of AP is = 5. And common difference of AP is d = a2 − a1 = 9 − 5 = 4. Let nth term of given AP is 81. i.e., an = 81 ⇒ a + ( n − 1)d = 81. [∵an = a + ( n − 1)d] ⇒ 5 + (n – 1)4 = 81 ⇒ 4(n –1) = 81 – 5 = 76 ⇒ n – 1 = \(\frac{76}{4}\) = 19 ⇒ n = 19 +1 = 20. Hence, 81 is 20th term of AP. |
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