Find the number of terms of the A.P. -12,-9, -6, ...,if 1 is added to each term of this A.P., then find the sum of the A.P. thus obtained.
[CBSE 2013]
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5
Solution :
Well, the first term that appears in both of them is 21.
Since,
4 and 5 (the forward differences) are coprime, see that the sequences will only be equal every time the sequence increases by 4×5 =20.
- In other words,
21+41+61+81+…
401=∑n
=120020n+1
=200(201)+200
=200(202)
=40400
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