# Trigonometry - Associated Acute Angles

1. Dec 19, 2013

### BOAS

Hello,

i'm doing some revision and working through the textbook my course follows and have a small problem.

The question is an incomplete table of values for sinθ, cosθ, tanθ, $\alpha$ and θ. I have to work out the blanks using what is given.

I thought I knew the correct method of doing this, and I got all the answers correct except for the last two lines where I got the value correct, but did not see why I needed to include a ± sign.

I'll show what I did for one of the lines, and hopefully someone can see why i'm not realising the values can be negative or positive.

1. The problem statement, all variables and given/known data

sinθ =
cosθ =
tanθ =
$\alpha$ = 45°
θ =

3. The attempt at a solution

I worked out sin, cos and tan for this value of alpha and they were all positive, so I thought I could assume, based on the CAST diagram, that the associated acute angle $\alpha$, must lie in the 1st quadrant. Therefore θ = 45°

However, my textbook says all the trig values I obtained can be ± and θ = 45° or 135°

What am I missing?

2. Dec 19, 2013

### Staff: Mentor

How are α and θ related?
Are you given a picture of the triangle?
You haven't provided enough information for us to be able to help you.

3. Dec 19, 2013

### BOAS

Sorry, I didn't stop to think whether my explanation involving α would make sense.

I'll do my best to explain it, but it's easier with a set of axes in front of you.

Take o to be the origin, and draw a line op at say 45° to the x axis. Directly below p, on the x axis, we label the point q to construct a right angled triangle. The anticlockwise direction is taken as +ve. We can say that α is the associated acute angle for θ here because it lies in the 'first quadrant'. They are numbered from 1-4 anticlockwise.

Now suppose you rotate the line op to a position of 135°, p lies in the 'second' quadrant and θ = 135°, but the associated acute angle α is 45° (the triangle opq)

I hope that makes enough sense to see what my question is getting at.

4. Dec 19, 2013

### tiny-tim

i don't get it

if OP is 120°, what is α?​

5. Dec 19, 2013

### BOAS

If the line op is rotated 120° anticlockwise, then θ is 120° and α is the acute angle made with the x axis, so it's 60°.

EDIT:

This is in relation to the CAST diagram, that tells us which trig ratios are positive in which quadrant.

6. Dec 19, 2013

### tiny-tim

I see.

And what about 240° and 300° … are they 60° or -60° ?

7. Dec 19, 2013

### BOAS

In the first quadrant α = θ - 360°
second quadrant α = 180° - θ
Third quadrant α = θ - 180°
Fourth quadrant α = 360° - θ

So, if θ = 240°, the line op lies in the third quadrant and α = 60°.

If θ = 300°, the line op lies in the fourth quadrant and α = 60°

I'm pretty sure it's setup so that α is always positive.

8. Dec 19, 2013

### tiny-tim

ok, then if θ = 45° or 135°, then both cosθ and tanθ can be ± (though sinθ can only be +)

however, on your explanation of α, i don't see why θ can't be 225° or 315°

9. Dec 19, 2013

### BOAS

I'm sorry, this whole question is due to me mis-reading the answer at the back of the book.

It was written as θ = ±45° or ±135° and I didn't see the signs in front of the values it gave for θ. I have it all making sense now.