I Have Questions. You've Got The Answers 1

Let $a_{n}$ be a sequence of real numbers defined by the recursion $a_{n+2} = a_{n+1} - a_{n}$ for all positive integers $n$ . If $a_{2013} = 2015$ , find the value of $a_{2017} - a_{2019} + a_{2021}$ .

$a. 2015$

$b. -2015$

$c. -4030$

$d. 4030$

Easy Math Editor

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`2 \times 3`	$2 \times 3$
`2^{34}`	$2^{34}$
`a_{i-1}`	$a_{i-1}$
`\frac{2}{3}`	$\frac{2}{3}$
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Math	Appears as
Remember to wrap math in `\(` ... `\)` or `\[` ... `\]` to ensure proper formatting.
`2 \times 3`	$2 \times 3$
`2^{34}`	$2^{34}$
`a_{i-1}`	$a_{i-1}$
`\frac{2}{3}`	$\frac{2}{3}$
`\sqrt{2}`	$\sqrt{2}$
`\sum_{i=1}^3`	$\sum_{i=1}^3$
`\sin \theta`	$\sin \theta$
`\boxed{123}`	$\boxed{123}$