Inverse functions and substitution

Hi, I am having trouble finding resources on how to apply the properties of inverse functions arcsin, arccos and arctan, as well as how to use substitution to solve equations such as:

Everything I come across seems to go over the properties of the inverse functions and graph them etc. which I already understand; however, I am still confused as to how I would apply the properties to solve trig equations.

How should I go about solving the above problem and can anyone direct me to some more examples?

Thanks.

Re: Inverse functions and substitution

Quote:

Originally Posted by

**terrorsquid** Hi, I am having trouble finding resources on how to apply the properties of inverse functions arcsin, arccos and arctan, as well as how to use substitution to solve equations such as:

Everything I come across seems to go over the properties of the inverse functions and graph them etc. which I already understand; however, I am still confused as to how I would apply the properties to solve trig equations.

How should I go about solving the above and can anyone direct me to some more examples?

Thanks.

Solve for cos(x), then solve for x!

Since 5 + 12 = 17, we have

17cos(x) = 3

cos(x) = 3/17

x = arccos(3/17) and 2pi - arccos(3/17)

Now if the problem had been

5cos(x) + 12sin(x) = 3, then we would take a different (and more interesting) approach...

Re: Inverse functions and substitution

Are you also searching for other types of goniometric equations? ...

Re: Inverse functions and substitution

Quote:

Originally Posted by

**TheChaz** Now if the problem had been

5cos(x) + 12sin(x) = 3, then we would take a different (and more interesting) approach...

This is what I meant to write and is what I have been trying to find examples of but I can only seem to find definitions of inverse functions or trig substitution used to integrate and not for solving trig equations.

The only thing I seem to spot is that the squares of the coefficients on the LHS add up to a clean 169 so if you divided both sides by 13, you would you have the squares of the coefficients = 1.

How would you apply substitution or the properties of inverse functions to the eqation:

Never thought I'de like trig, but this has got me interested again :D

Re: Inverse functions and substitution

Quote:

Originally Posted by

**Siron** Are you also searching for other types of goniometric equations? ...

If 5cos(x) + 12sin(x) = 3 is a goniometric equation then yes :D

Re: Inverse functions and substitution

I think you can try something with the t-formulas, let .

Are you familiar with this? ...

Re: Inverse functions and substitution

Quote:

Originally Posted by

**terrorsquid** This is what I meant to write and is what I have been trying to find examples of but I can only seem to find definitions of inverse functions or trig substitution used to integrate and not for solving trig equations.

The only thing I seem to spot is that the squares of the coefficients on the LHS add up to a clean 169 so if you divided both sides by 13, you would you have the squares of the coefficients = 1.

How would you apply substitution or the properties of inverse functions to the eqation:

Never thought I'de like trig, but this has got me interested again :D

You are on the right track! And while (5, 12, 13) is a pythagorean triple, you can perform this with any values of A and B for

. Just divide by

So here we go.

Dividing by 13, we have

Let t be the angle with sine 5/13 (and hence cosine of 12/13).

Then our equation becomes

The left hand side is the sine angle sum...

arcsine

find the value of t

etc.

Re: Inverse functions and substitution

Ah, ok. I got to the point where I could see that with the new coefficients, the equation was obviously on the unit circle; however, when I drew in the coordinates 5/13 and 12/13, I didn't know which was cos(x) and which was sin(x) (x,y) on the circle. I tried once just guessing and put +5/13 on the x axis and +12/13 on the y axis. Therefore, I had:

LHS = cos angle difference:

Is it that the same answer? how do I know which coefficient to use on which axis?

Thanks.

Re: Inverse functions and substitution

Quote:

Originally Posted by

**Siron** I think you can try something with the t-formulas, let

.

Are you familiar with this? ...

I have something written down:

I don't know where they came from though and don't understand their use; the only identity I know for and would be the double angle identities.

Re: Inverse functions and substitution

That's ok, now you can wright the equation as:

Solve this equation for and afterwards do the back-substitution.

Re: Inverse functions and substitution

Quote:

Originally Posted by

**terrorsquid**

I prefer sine, so I chose it as such.

They are the same, but don't forget that there are infinite (though coterminal) solutions - and that arccos(5/13) = arcsin(12/13) and vice versa.

Re: Inverse functions and substitution

Ok, cool. Would the function still have period ? and therefore an infinite amount of answers at where (I'm a little confused on how to determine the solution set).

Also, with the other method I get:

So, I subbed the two values in for cos(x) and solved for sin(x) and vice versa to get 4 different answers. approx ~ 9,-9, 35 and 144. Again, I don't know how to define the infinite set of answers or, if there was a given restriction, how to discern which are valid answers.

Re: Inverse functions and substitution

Is this the you become by using the t-formulas?

The only thing you've to do now is the back-substitution:

Can you continue now? (as The Chaz noticed there are infinite solutions)

Re: Inverse functions and substitution

Quote:

Originally Posted by

**Siron** Is this the

you become by using the t-formulas?

The only thing you've to do now is the back-substitution:

Can you continue now? (as The Chaz noticed there are infinite solutions)

I'm ok solving the equations now, I am just having trouble visualising which quadrants the function appears in because there is a sin and a cos in it. If it was just cos(x) = 1/2, I could just think cos(x) is positive therefore it is in the 1st and 4th quadrants and then think it is either 60 or 360-60 (and 2pi k multiples of each). Or if it was sin(x) = 0 I can imagine the sin curve and know that it will be 0 at 0 and any integer multiple of pi. But how do you interpret a heterogeneous function like this one and know where it will reoccur?

Re: Inverse functions and substitution

Quote:

Originally Posted by

**terrorsquid**

Say, for example, the above was between and what would you be thinking when figuring out the solution set?