Step by step
The question
Question: graph y = −x².
−x² means −1x², so a = −1.
a is negative, so the U opens down, like a hill.
1 Find the vertex
The vertex is the turning point of the curve.
Its x is minus b/2a.
In y = ax² + bx + c, here a = −1 and b = 0.
Minus 0 is 0.
Plus times minus is minus: 2 times −1 is −2.
0 divided by −2 is 0.
To get y, put x = 0 in.
We get y = 0.
So the vertex is (0, 0).
2 Make a table
We make a table around the vertex: x from −2 to 2.
At x = 0, the vertex: y = 0.
Now the right side.
Put x = 1 in.
We get y = −1.
Next.
Put x = 2 in.
We get y = −4.
The curve is the same on both sides of the vertex, like a mirror.
So x = −1 gives the same y as x = 1.
And x = −2 is the same as x = 2.
3 Plot the points
Draw the grid.
The x-axis goes across.
The y-axis goes up and down.
They meet at 0.
That point is called the origin.
Put the points from the table on the grid.
The mirror line goes through the vertex: x = 0.
4 Draw the curve
Join the points with a smooth curve, not straight lines.
This upside-down U is a parabola too.
5 Crossing the x-axis
The curve just touches the x-axis at x = 0, at the vertex.
6 Check
Let's check the mirror.
Put x = −2 straight in.
We get −4, the same as in the table.
Let's look back.
First: find the vertex.
Then: make a table.
Then: plot the points.
Then: draw the curve.
Then: crossing the x-axis.
Last: check.