Can you check if this is right?

Question

help_people · Answer

The lengths of two sides of a triangle are shown below:

Side 1: 3x2 − 4x − 1
Side 2: 4x − x2 + 5

The perimeter of the triangle is 5x3 − 2x2 + 3x − 8.

Part A: What is the total length of the two sides, 1 and 2, of the triangle? 

Part B: What is the length of the third side of the triangle? 

Part C: Do the answers for Part A and Part B show that the polynomials are closed under addition and subtraction? Justify your answer.

help_people · Answer

Part A: To get the total length of the two sides you ahem to add them together: 

3x^2 - 4x -1
-x^2+4x+5
--------------
2x^2+4

Part B: To find the length of the third side of the triangle you will add the perimeter and the total length of the two sides together: 

5x^3 - 2x^2 + 3x -8
-(2x^2 +4) 
----------------------
5x^3 - 4x^2 + 3x -12

help_people · Answer

idk what they're asking for in pt c?

hartnn · Answer

perimeter = side 1 + side 2 + side 3

so side 3 = perimeter - (side 1 + side 2)

hartnn · Answer

lets first correct the answer of part B :)

hartnn · Answer

To find the length of the third side of the triangle you will subtract  the total length of the two sides from the perimeter

help_people · Answer

woat so part b is wrong?

hartnn · Answer

yes, part B is wrong.

help_people · Answer

@hartnn

help_people · Answer

ok

hartnn · Answer

the  words in part B are incorrect, 
the answer is correct, because you indeed subtracted the answer from part A.

just replace the description in your part B.

------
Part B: To find the length of the third side of the triangle you will subtract  the total length of the two sides from the perimeter.

5x^3 - 2x^2 + 3x -8
-(2x^2 +4) 
----------------------
5x^3 - 4x^2 + 3x -12

help_people · Answer

ok

hartnn · Answer

Part C: 
Polynomials are closed under addition if you get a polynomial by adding polynomials.
Did you get a polynomial answer for part A?

Polynomials are closed under subtraction if you get a polynomial by subtracting polynomials.
Did you get a polynomial answer for part B?

help_people · Answer

yes and yes right?

hartnn · Answer

good!
since we get a polynomial after adding and subtracting polynomials, 
we can conclude,
that the polynomials are closed under addition and subtraction.

help_people · Answer

thank you! may you check 2 more?

help_people · Answer

@hartnn ??