Step by step
The question
Question: graph y = cos(x − 90°).
The x values are in degrees.
1 Read A, B, C, D
Every cosine graph can be written like this: y = A cos(B(x − C)) + D.
Each letter does one job.
Here A is 1, the number in front.
B is 1, the number in front of x.
x minus C: C is 90°.
Nothing is added at the end, so D is 0.
2 Find the amplitude
The amplitude is how far the wave goes up and down from its middle line.
It is |A|: 1.
3 Find the period
One full wave of cos x is a full turn: 360° long.
B squeezes the wave: the period is 360° over B.
B is 1.
So the period is 360°: one full wave every 360°.
4 Find the shifts
C moves the wave sideways.
x − 90° moves it right: the wave starts at x = 90°.
Careful!
A minus inside the bracket moves the wave right, not left.
At x = 90°, the inside is 0: that is where the wave starts.
5 Make the key points
We use a table with five key points: one full wave.
x starts at 90°.
Each next x is a quarter of the period further: 90°.
A quarter of 360° is 90°.
The cos part follows the same pattern in every cosine wave: 1, 0, −1, 0, 1.
Each y is A times that: the same numbers.
6 Draw the graph
Draw the grid.
The x-axis is in degrees: a mark every 90°.
Plot the key points from the table.
Join them with a smooth wave: round at the top and the bottom, never pointed.
The wave keeps going the same way, both ways.
7 Check
Let's check one point from the table: x = 90°.
Put it in the equation.
The inside is 0°, and cos 0° is 1.
So y is 1, like in the table.
So the graph is right!
Let's look back.
First: read A, B, C, D.
Then: find the amplitude.
Then: find the period.
Then: find the shifts.
Then: make the key points.
Then: draw the graph.
Last: check.