# Math Help - sequences

1. ## sequences

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2. Originally Posted by mathh18
can anyone help write a closed formula for any of these?
1, 7, 13, 19, 25...

2, 10, 50, 250, 1250...

-3, 5, 17, 33, 53...

i'm stuck on all three of these, any help would be great thank you so so so much!
1) common difference is 6 - try $a_n = 6n+$k and find k that works.

2) Dividing shows the same number - 5, try $a_n = c_1 5^n + c_2$

3) the difference is 8, 12, 16, 20 and then again is 4 which suggests a formula of the form $a_n = pn^2+qn+r$ - find p,q and r.