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# Problems involving Arithmetic Progressions watch

1. 4. The first, second and fourth terms of an Arithmetic Progression are x + 1, 2x - 1, and 2x + 5.

Sn formula is Sn = 1/2n(2a+d(n-1))

I'm completely lost...
a = x + 1

2nd term = a + d
2x - 1 = x + 1
x = 2 but it says x = 5
Then others I'm clueless how to find.
2. (Original post by ckfeister)
4. The first, second and fourth terms of an Arithmetic Progression are x + 1, 2x - 1, and 2x + 5.

Sn formula is Sn = 1/2n(2a+d(n-1))

I'm completely lost...
a = x + 1

2nd term = a + d
2x - 1 = x + 1
x = 2 but it says x = 5
Then others I'm clueless how to find.
What is your d? You didn't include that in the bold line. Find d by doing and then you may proceed.

Also what are you trying to find? If it's not the sum then that formula is useless.
3. (Original post by RDKGames)
What is your d? You didn't include that in the bold line. Find d by doing and then you may proceed.

Also what are you trying to find? If it's not the sum then that formula is useless.
I have to find x,a,d
x = 5, a = 6, d = 3, but don't know how to find them.
4. (Original post by ckfeister)
I have to find x,a,d
x = 5, a = 6, d = 3, but don't know how to find them.
So

so we get the difference in terms of x which is

Then express the fourth term in terms of the first one and a certain multiple of the difference.
5. (Original post by RDKGames)
So

so we get the difference in terms of x which is

Then express the fourth term in terms of the first one and a certain multiple of the difference.
What did you remove? I was eating came back and it was gone.
6. bump
7. (Original post by ckfeister)
What did you remove? I was eating came back and it was gone.
Error.

Simply express the fourth term since you know the formula and solve for x.
8. (Original post by RDKGames)
Error.

Simply express the fourth term since you know the formula and solve for x.
Ohhh I've been using Sn not Un, thanks.

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