# I have to graph trig functions, but when I graph it, it is always wrong. I know how to find the period and the altitude, but where do I start, and how do I actually graph it?

## Expert Answers We will use a sine function as an example, but the method is general:

Suppose you are asked to graph `y=-2sin(1/3*pi(x+2))-2 `

This is of the form `y=asin(b(x-h))+k `

a: a is the amplitude. This is the maximum distance the graph takes from the midline. Note if a<0 the graph is reflected over the midline.

b: b yields the period; the period p is found by `p=(2pi)/b ` . (If you were graphing tangent or cotangent the numerator is pi.) This gives the horizontal distance required for the graph to begin repeating. (If b<0 the graph is reflected over a vertical line. It is often easier to rewrite the original function since sin(-x)=-sin(x) and cos(-x)=cos(x).)

h: h is the horizontal translation or phase shift. If h>0 shift to the right, if h<0 shift to the left h units.

k: y=k is the midline.

So for our example a=-2, b=1/3pi, h=-2 and k=-2

The graph is a transformed sine wave: the amplitude is 2, the graph is reflected over the horizontal, the period is 6, the graph is shifted left 2 units, and the midline is y=-2.

Here is a graph of each of the transformations with the final answer in green:

Red is the original sine, orange has amplitude 2 and has been reflected, blue has the change in period, purple has the phase shift, and green shifts vertically (moved the midline.)

Approved by eNotes Editorial Team # I have to graph trig functions, but when I graph it, it is always wrong. I know how to find the period and the altitude, but where do I start, and how do I actually graph it?

First I think you mean amplitude versus altitude.  When I graph trig functions I first look to see if there is a phase shift or horizontal shift.  Depending on the type of function (sin, cos, tan, etc)  I first usually find my 5 values for when each of these are either 0,1, -1, or undefined.

For instance if I was graphing `y = sin x`

`sin 0 = 0 `

`sin (pi/2)= 1`

`sin pi = 0`

`sin (3pi/2)=-1`

`sin 2pi =0`

From here either shift horizontally left or right, and then up or down based on vertical shift.

Approved by eNotes Editorial Team

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