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Omni-MATH / Let q be a real number. Gugu has a napkin with ten distinct real numbers written on it, and he writes the following three lines of real numbers on the blackboard: [list] [*]In the …

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problem

Let qq be a real number. Gugu has a napkin with ten distinct real numbers written on it, and he writes the following three lines of real numbers on the blackboard: [list] [*]In the first line, Gugu writes down every number of the form a−ba-b, where aa and bb are two (not necessarily distinct) numbers on his napkin. [*]In the second line, Gugu writes down every number of the form qabqab, where aa and bb are two (not necessarily distinct) numbers from the first line. [*]In the third line, Gugu writes down every number of the form a2+b2−c2−d2a^2+b^2-c^2-d^2, where a,b,c,da, b, c, d are four (not necessarily distinct) numbers from the first line. [/list] Determine all values of qq such that, regardless of the numbers on Gugu's napkin, every number in the second line is also a number in the third line.
Plain-text mathematical notation (without MathML)
Let q be a real number. Gugu has a napkin with ten distinct real numbers written on it, and he writes the following three lines of real numbers on the blackboard:
[list]
[*]In the first line, Gugu writes down every number of the form a−b, where a and b are two (not necessarily distinct) numbers on his napkin.
[*]In the second line, Gugu writes down every number of the form qab, where a and b are
two (not necessarily distinct) numbers from the first line.
[*]In the third line, Gugu writes down every number of the form a²+b²−c²−d², where a,b,c,d are four (not necessarily distinct) numbers from the first line.
[/list]
Determine all values of q such that, regardless of the numbers on Gugu's napkin, every number in the second line is also a number in the third line.
Original LaTeX notation
Let $q$ be a real number. Gugu has a napkin with ten distinct real numbers written on it, and he writes the following three lines of real numbers on the blackboard:
[list]
[*]In the first line, Gugu writes down every number of the form $a-b$, where $a$ and $b$ are two (not necessarily distinct) numbers on his napkin.
[*]In the second line, Gugu writes down every number of the form $qab$, where $a$ and $b$ are
two (not necessarily distinct) numbers from the first line.
[*]In the third line, Gugu writes down every number of the form $a^2+b^2-c^2-d^2$, where $a, b, c, d$ are four (not necessarily distinct) numbers from the first line.
[/list]
Determine all values of $q$ such that, regardless of the numbers on Gugu's napkin, every number in the second line is also a number in the third line.

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