OG 2017 Online Two Part Analysis Problem

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OG 2017 Online Two Part Analysis Problem

by jloock524 » Tue Jun 27, 2017 12:15 pm
attached is an IR Two Part Analysis question; sequencing is the topic. Please explain how to complete this problem. I am having trouble understanding the explanation the OG gives.

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by ceilidh.erickson » Tue Jul 04, 2017 9:50 am
Here, we can translate the sequence notation Image
as the following:
For any given nth term (Sn), the value of the term two places above it (S(n+2)) will be equal to the nth term squared minus the value of the term one above the nth term (S(n-1)).

Or in other words...
Image

Since S4 depends on knowing the values of both S2 and S3, and S3 depends on knowing the values of S1 and S2, let's begin by testing the possible given values of S1:

Image

Now, generate the value of S3 for each by squaring S1 then subtracting S2:
Image

Now, generate S4 for each by squaring S2 then subtracting S3:
Image

Now we need to find a value for S1 that would have a matching value for S4.
Image

Since -142, -47, and 1 are not options for S4, eliminate -12, -7, and -1 as options for S1. S4 = 0 is an option... but we're told to select 2 different values, so this must not be the answer.
The only valid match is:
S1 = -3
S4 = -7
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education