Identify the axis of symmetry, create a suitable table of values, then sketch the graph (including the axis of symmetry).
y = –x² + 3x – 3
First make the table:
x|-1| 0| 1| 2| 3| 4
y|-7|-3|-1|-1|-3|-7
Then we'll plot those points
Next we'll calculate the axis of symmetry:
Rule:
The axis of syymetry of the graph whose equation is
y = Ax² + Bx + C
is the vertical line whose equation is
x =
You may not have learned about the vertex, but I'll
throw that in too.
Rule:
The vertex is the point
(, ), where D = B²-4AC
(D is called the "discriminant")
So in the case of
y = –x² + 3x – 3
A = -1, B = 3, C = -3
so the axis of symmetry is the vertical line
whose equation is
x =
or
x =
x =
x =
So we'll sketch in the
axis of symmetry:
and to find the vertex,
D = B² - 4AC
D = 3² - 4(-1)(-3)
D = 9 - 12
D = -3
So the vertex is the point
(, or
(, , or
(, , or
(, ),
So we'll plot the vertex point too:
Now we'll sketch in the graph, which is
a parabola:
Edwin