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# C2 Sequences and recursive definitions... watch

1. I have the sequence Ur = 1/2 r (r + 1)

Somehow the sequence is 1, 3, 6, 10, 15.

I understand how to get the recursive definition of Ur + 1 = Ur + r + 1 but don't have a clue with the first bit.

Any help would be appreciated.
2. All its saying is The number you start with in the sequence is 1. So a is 1
Ur is one
So 1/2(1)(1+1) and you end up with =3
then next number is 2, so 2 is r, with U2 looking like 1/2(2)(2+1) which gives you 3

stick some more values in for 'r' and see what you get

I think you may not like r as its used in geometric sequences as the common ratio?
3. When r = 1, 1/2 times 1 times 2 is 1
When r = 2, 1/2 times 2 times 3 is 3
When r = 3, 1/2 times 3 times 4 is 6

etc
4. (Original post by dexter -1)
All its saying is The number you start with in the sequence is 1. So a is 1
Ur is one
So 1/2(1)(1+1) and you end up with =3
then next number is 2, so 2 is r, with U2 looking like 1/2(2)(2+1) which gives you 3

stick some more values in for 'r' and see what you get

I think you may not like r as its used in geometric sequences as the common ratio?
thanks
5. (Original post by stevencarrwork)
When r = 1, 1/2 times 1 times 2 is 1
When r = 2, 1/2 times 2 times 3 is 3
When r = 3, 1/2 times 3 times 4 is 6

etc
thanks

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