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Question
Which term of the A.P. 3, 8, 13, 18, ..., is 78?
Solution
3, 8, 13, 18, …
For this A.P.,
a = 3
d = a2 − a1
= 8 − 3
= 5
Let nth term of this A.P. be 78.
an = a + (n − 1) d
78 = 3 + (n − 1) 5
75 = (n − 1) 5
(n − 1) = 15
n = 16
Hence, 16th term of this A.P. is 78.
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