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Answer:
A. 2x^2 +4
B. 5x^3 -4x^2 +3x -12
C. addition and subtraction of polynomials result in polynomials (closed)
Explanation:
A. The total of the two side lengths is ...
(3x^2 -4x -1) +(-x^2 +4x +5) = (3 -1)x^2 +(-4+4)x +(-1+5) = 2x^2 +4
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B. The length of the third side is the difference between the total for all three sides and the total for the given two sides:
(5x^3 -2x^2 +3x -8) - (2x^2 +4) = 5x^3 +(-2-2)x^2 +3x +(-8-4)
= 5x^3 -4x^2 +3x -12
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C. Addition of the two polynomials results in a polynomial. Subtraction of the two polynomials results in a polynomial. There is nothing here to indicate the set of polynomials is not closed under addition and subtraction.