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Determine whether the following are subspaces of P 4 (Be Careful!) (P 4 is defin

ID: 2939971 • Letter: D

Question

Determine whether the following are subspaces of P4 (Be Careful!)
(P4 is defined as the set of all polynomialsof degree less than four)

The set of all polynomials of degree 3
and
The set of all polynomials p(x) in P4 suchthat p(0) = 0

This one is giving me quite a bit of trouble! I want to beable to explain this and work it out, rather than be able to seethat the first is not a subspace and the second is a subspace.Thank you very much.
(P4 is defined as the set of all polynomialsof degree less than four)

The set of all polynomials of degree 3
and
The set of all polynomials p(x) in P4 suchthat p(0) = 0

This one is giving me quite a bit of trouble! I want to beable to explain this and work it out, rather than be able to seethat the first is not a subspace and the second is a subspace.Thank you very much.
The set of all polynomials of degree 3
and
The set of all polynomials p(x) in P4 suchthat p(0) = 0

This one is giving me quite a bit of trouble! I want to beable to explain this and work it out, rather than be able to seethat the first is not a subspace and the second is a subspace.Thank you very much.

Explanation / Answer

The set of all polynomials of degree 3 is not a subspace of P4 because the first and second conditions of a subspace are not satisfied. To see this consider the following elements of the set given in (b): x3 + x2 + x and ?x3 + x2 + x. We can see the first condition is not satisfied because if we take the scalar zero and the first element given above we obtain: 0(x3 + x2 + x) = 0. Clearly zero is not a polynomial of degree 3 and the first condition of a subspace is not satisfied. When the two elements above are added together we obtain: (x3 + x2 + x) + (?x3 + x2 + x) = 2x2 + 2x. Clearly 2x2 + 2x is not a polynomial of degree 3 and the second condition of a subspace is not satisfied.

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