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For each positive integer n, the quantity s_n is defined such that \(\mathrm{s_{n+2} = (s_n)^2 - s_{n+1}}\). In addition, s_2...

GMAT Two Part Analysis : (TPA) Questions

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Two Part Analysis
Quant - Fitting Values
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For each positive integer \(\mathrm{n}\), the quantity \(\mathrm{s_n}\) is defined such that \(\mathrm{s_{n+2} = (s_n)^2 - s_{n+1}}\). In addition, \(\mathrm{s_2 = 1}\).

Select values for \(\mathrm{s_1}\) and \(\mathrm{s_4}\) that are jointly compatible with these conditions. Make only two selections, one in each column.

s 1

s 4

-12

-7

-3

-1

0

Solution

Phase 1: Owning the Dataset

Understanding the Sequence

We have a recursive sequence where:

  • \(\mathrm{s_{n+2} = (s_n)^2 - s_{n+1}}\)
  • \(\mathrm{s_2 = 1}\) (given)

Let's create a visual showing how the sequence builds:

\(\mathrm{s_1 \rightarrow s_2 \rightarrow s_3 \rightarrow s_4}\)
\(\mathrm{? \quad 1 \quad ? \quad ?}\)

Phase 2: Understanding the Question

Finding the Relationships

Using our recursive formula, let's express \(\mathrm{s_3}\) and \(\mathrm{s_4}\) in terms of \(\mathrm{s_1}\):

For \(\mathrm{s_3}\):

  • \(\mathrm{n = 1: s_3 = (s_1)^2 - s_2 = (s_1)^2 - 1}\)

For \(\mathrm{s_4}\):

  • \(\mathrm{n = 2: s_4 = (s_2)^2 - s_3 = 1 - s_3}\)
  • Substituting \(\mathrm{s_3: s_4 = 1 - ((s_1)^2 - 1) = 2 - (s_1)^2}\)

Key Insight: We need to find values from the answer choices where \(\mathrm{s_4 = 2 - (s_1)^2}\)

Phase 3: Finding the Answer

Systematic Checking

Let's check each possible value of \(\mathrm{s_1}\):

If \(\mathrm{s_1 = -12 \rightarrow s_4 = 2 - (-12)^2 = 2 - 144 = -142}\)
Is -142 in our choices? No, continue.

If \(\mathrm{s_1 = -7 \rightarrow s_4 = 2 - (-7)^2 = 2 - 49 = -47}\)
Is -47 in our choices? No, continue.

If \(\mathrm{s_1 = -3 \rightarrow s_4 = 2 - (-3)^2 = 2 - 9 = -7}\)
Is -7 in our choices? Yes! ✓
Stop here - we found our answer.

Verification

Let's verify our answer works:

  • \(\mathrm{s_1 = -3}\)
  • \(\mathrm{s_2 = 1}\) (given)
  • \(\mathrm{s_3 = (s_1)^2 - s_2 = 9 - 1 = 8}\)
  • \(\mathrm{s_4 = (s_2)^2 - s_3 = 1 - 8 = -7}\)

Phase 4: Solution

Final Answer:

  • Statement 1 (\(\mathrm{s_1}\)): -3
  • Statement 2 (\(\mathrm{s_4}\)): -7

These values satisfy the recursive relationship \(\mathrm{s_{n+2} = (s_n)^2 - s_{n+1}}\) with \(\mathrm{s_2 = 1}\).

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