Explanation:
a)
y = 2x³ + 4x
a polynomial of 3rd degree cannot have an axis of symmetry.
it can only have a point of symmetry, like in this case.
(0, 0) is the point of symmetry, because due to the fact there is no squared term, any negative x values create exactly the same values (in absolute values) as their positive counterparts - just negative.
the vertex of a polynomial of 3rd degree is the point where the function changes directions.
that point for x³ is the origin, as it is for x. the factors only stretch the graph, but the vertex stays the same.
only if we had a term like (x+a)³ in the polynomial, this would shift the vertex.
so, the point of symmetry and the vertex are the same :
(0, 0).
b)
y = -x² + 4x - 5
the vertex form of a quadratic function is
f(x) = a(x − h)² + k
with (h, k) is the vertex.
x = h is the axis of symmetry.
we use "complete the square" to get it into that form.
-x² + 4x - 5 = -(x² - 4x + 5) = -(x - 2)² - 1
therefore,
y = -(x - 2)² - 1
h = 2, k = -1
the vertex is (2, -1)
the axis of symmetry is x = 2