This is closed under addition because the sum of the polynomial produces another polynomial.
What is a polynomial?
A polynomial is an expression that involves the addition, subtraction and multiplication of variables.
Polynomials will be closed under an operation if the operation produces another polynomial.
(3x⁴ + 5x - 7) + (2x - (2x)² + 4x - 1) + [(-8x)⁻² - (3x)³ + 7]
= 3x⁴ - 27x³ - 4x² + 9x - 1 + (-8x)⁻²
This is closed under addition because the sum of the polynomial produces another polynomial.
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