Asked by Marianne

(This is a question where we have to fill in the blank)
So:
2 3 5 ? 12
11 16 26 41 ?

It's a number sequence question and I need to fill in the ? as well as write a rule in words. PLEASE HELP!(And the top number has to relate with the bottom number)
Answered by Quidditch
For determining sequences a good start is to look at the difference between adjacent numbers.
For the sequence:
2, 3, 5, ?, 12
write the differences. This sequence is not a hard one.
Answered by Marianne
hm, 8?
Answered by Quidditch
Yes! Excellent!
Answered by Marianne
That's what I wrote. But my teacher circled it, meaning it's wrong. :(
Answered by Quidditch
Now for the second part, how does the second row relate to the first.

2, 3, 5, 8, 12
11, 16, 26, 41, ?

First I would check what would be added to the top row to get the bottom row. That would be 9, 13, 21, 33.
Those don't seem very related. What would be multiply the top row by to get close to the bottom row? Divide the bottom row number by the top row number. What do you get? What can you do to make that acutally be the bottom row number?
Answered by Marianne
well, 5 would be the closest. I'm still quite confuse with the last statement, "What can you do to make that actually be the bottom row number?"
Answered by Quidditch
I am not sure why your teacher circled it. When the problem is solved, 8 fits into the sequence top and bottom.
Answered by Reiny
I agree with 8 as well.
Have your teacher explain to you why it would not be 8.

Did you notice that for the second one, your difference are 5, 10, 15, .... ?
Answered by Quidditch
If you multiply the top row number by 5 and then add 1, you get the bottom row number.
Answered by Marianne
yes exactly!
Perhaps my teacher corrected it wrong...
Answered by Marianne
Can you help me find an algebraic rule for this?:
2 3 5 x 20
20 29 47 83 y

and x i got 9
and y i got 158
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