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# Arithmetic Progressions question (AS Level) Watch

1. Right, after about 5 minutes of googling I cannot find what I'm looking for.
Can anyone help me?
Is there a quick way of finding the term which gives a certain number in a sequence?
For example

1 + 3 + 5 + 7 + ... + 47 + 49

I know the term that gives 49 is T25 (after a lot of messing about)

But I was wondering if there is a straight method to finding it.
Some rearrangement of Tn = a + (n - 1)d perhaps?

2. if you know a and d you can do it for the example you gave

49 = 1 + (n - 1)2

49 = 1 + 2n - 2

50 = 2n

25 = n
3. Wow. That's so obvious. Now I feel dumb!

Thank you so much! Saved me from wasting a lot of time there.
4. (Original post by HamLetuceTomato)
Wow. That's so obvious. Now I feel dumb!

Thank you so much! Saved me from wasting a lot of time there.
You're welcome and don't feel dumb, everyone does that, i was just looking at conditional probability going...wtf and then i saw a video with someone who explained it and made it look so easy and I'm like, how did I not get that P

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