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Question
Decide whether the following sequence is an A.P., if so find the 20th term of the progression:
–12, –5, 2, 9, 16, 23, 30, ..............
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Solution
Here a = t1 = first term = – 12, t2 = – 5,
Common difference = d = t2 – t1
d = – 5 – (– 12)
= – 5 + 12
∴ d = 7
We know that, tn = a + (n – 1)d
Here, n = 20, a = – 12, d = 7
∴ t20 = – 12 + (20 – 1)7
= – 12 + 133
t20 = 121
∴ 20th term of the progression is 121.
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