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Question
Find the 20th term of the AP 9, 13, 17, 21,....
Sum
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Solution
The given AP is 9, 13, 17, 21,....
First term, a = 9
Common difference, d = 13 – 9 = 4
nth term of the AP,
an = a + (n – 1)d
= 9 + (n – 1) × 4
∴ 20th term of the AP,
a20 = 9 + (20 – 1) × 4
= 9 + 76
= 85
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