The general form of a quadratic function is y=ax2+bx+c. Investigate if the functions are in this form.
The function has two x2 terms. Thus, they can be added. y=x2+3x2⇔y=4x2. The function does not have an x-term or constant but it simply means that these terms are zero, so y=4x2+0x+0. Therefore, A is a quadratic function.
The function contains an x2-term, but also the term x0.5. Hence, it does not fit the form y=ax2+bx+c, and is not a quadratic function.
We see directly that the function is not a quadratic function because it contains a variable that is raised to 3.