ISEE Upper Level Quantitative : Data Analysis

Study concepts, example questions & explanations for ISEE Upper Level Quantitative

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Example Questions

Example Question #38 : Sets

Define a sequence as follows:

\(\displaystyle a_{1} = 3\)

\(\displaystyle a_{2} = 1\)

For all integers \(\displaystyle n > 2\)\(\displaystyle a_{n} = 3a_{n-1} - a_{n-2}\).

Which of the following expressions is equal to \(\displaystyle a_{1,000}\) ?

Possible Answers:

\(\displaystyle 8a_{998 } - 3 a_{997}\)

\(\displaystyle 8a_{998 } - a_{997}\)

\(\displaystyle 9a_{998 } - 2a_{997}\)

\(\displaystyle 9a_{998 } - a_{997}\)

\(\displaystyle a_{998 } -9a_{997}\)

Correct answer:

\(\displaystyle 8a_{998 } - 3 a_{997}\)

Explanation:

Setting \(\displaystyle n = 1,000\):

\(\displaystyle a_{n} = 3a_{n-1} - a_{n-2}\)

\(\displaystyle a_{1,000} = 3a_{1,000-1} - a_{1,000-2}\)

\(\displaystyle a_{1,000} = 3a_{999} - a_{998}\)

Similarly,

\(\displaystyle a_{999} = 3a_{998 } - a_{997}\)

Substituting:

\(\displaystyle a_{1,000} = 3 (3a_{998 } - a_{997}) - a_{998}\)

\(\displaystyle =9a_{998 } - 3 a_{997} - a_{998}\)

\(\displaystyle =8a_{998 } - 3 a_{997}\)

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