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## Geometry (all content)

### Course: Geometry (all content) > Unit 13

Lesson 5: Modeling with right triangles# Right triangle word problem

Sal solves a modeling problem where he finds the necessary angle to shoot at a vicious alien. Created by Sal Khan.

## Want to join the conversation?

- Could someone explain again what is the difference between tan and tan-1? Why did Sal use tan-1 instead of tan?(22 votes)
- hmm, it's very simple idea from functions. Function takes number from
**domain**(the set of numbers for which*you want the function to work on*) , works on it , and gives you number in the**codomain**or**range**(both essentially mean the numbers you get*from*function).**Inverse**function takes numbers from codomain or range of the function, works on it, and gives you numbers in domain of the function.

For our example, tan is function of our angle. it takes angles, which is domain, and gives you their tangents(opp/adj ratio if they are angle in right triangle), which is the range or codomain. Tan^-1, also known as**arctan**, takes the tangents,which is codomain of tan, and gives you the angles, which is domain of tan.

Hope that helps :D(52 votes)

- Hey why don't we just write tan^-1(6) instead writing tan^-1(324/54) . Would it be the same answer?(9 votes)
- Yes, you are correct. Most teachers would expect you to simplify the fraction.(12 votes)

- Isn't Cot is the inverse of Tan?(5 votes)
- No. The cotangent is the reciprocal, not the inverse of tangent. The inverse tangent is the arctangent.

NOTE: There is some unfortunate difficulty in notation. If the symbol ⁻¹ comes immediately after the name or symbol of a function, then it does NOT mean the reciprocal: it means the inverse function. But if ⁻¹ comes AFTER some constant, variable or other expression, then it means the reciprocal.

So, again, tan⁻¹ x is NOT equal to 1/tan x and so it is NOT the cotangent.(17 votes)

- What angle would he have to aim IF the laser beam was affected by gravity?(4 votes)
- Laser beams
*are*affected by gravity. But so minutely that the effect wouldn't appear in the second decimal place of the angle. So the answer would be the same.(16 votes)

- This question is mostly relying on calculator. Can you teach us how to do it using your brain, without paper, pencil, or calculator?(7 votes)
- Well technically if you can hold all the numbers in your head you can do anything using pencil/paper just in your head. So to do it by hand you need to apply Taylor series: Taylor series: http://www.khanacademy.org/math/integral-calculus/sequences_series_approx_calc/taylor-series/v/visualizing-taylor-series-approximations

If you can do this in your head you're on your way. Good luck.(9 votes)

- What is the difference between a right and a left triangle?(0 votes)
- Right triangles have a right angle. There are no left triangles.(11 votes)

- if there is a angle given and a side, how do I solve(10 votes)
- Technically, isn't the agent going to shoot the tip of the eiffel tower if he aims at 80.54 degrees? Yes, the alien is tiny but I think his height should be accounted for. Just a side thought.(5 votes)
- Or maybe we are try to blow the top of the tower off so that the alien falls. Of course, if we think to hard about this question, we will end up realizing things like the fact that the agent didn't need to know the angle to shoot, he could have just pointed the gun straight at the alien.(6 votes)

- (ANSWERED) Instead of using theta, could Sal use the right angle in the triangle for the equation?(5 votes)
- No. When using the trig ratios in right triangles, theta refers to an angle less than 90 degrees. If you used the hypotenuse, there would be ambiguity as to which side was adjacent, and because tan(90 deg) = undefined, and cos(90 deg) = 0. Both of those answers don't make sense in a solving a right triangle point of view.(7 votes)

- He's going to hit the alien's foot. Does this bother anyone else?(7 votes)

## Video transcript

A tiny but horrible
alien is standing at the top of the
Eiffel Tower-- so this is where the tiny but
horrible alien is-- which is 324 meters tall--
and they label that, the height of the Eiffel
Tower-- and threatening to destroy the city of Paris. A Men In Black-- or
a Men In Black agent. I was about to say maybe it
should be a man in black. A Men In Black agent is
standing at ground level, 54 meters across
the Eiffel square. So 54 meters from,
I guess you could say the center of the
base of the Eiffel Tower, aiming his laser
gun at the alien. So this is him
aiming the laser gun. At what angle should the
agent shoot his laser gun? Round your answer, if necessary,
to two decimal places. So if we construct a right
triangle here, and we can. So the height of this right
triangle is 324 meters. This width right over
here is 54 meters. It is a right triangle. What they're really
asking us is what is this angle right over here. And they've given us two
pieces of information. They gave us the side that
is opposite the angle. And they've given us the side
that is adjacent to the angle. So what trig function deals
with opposite and adjacent? And to remind
ourselves, we can write, like I always like
to do, soh, cah, toa. And these are really
by definition. So you just have to know this,
and soh cah toa helps us. Sine is opposite
over hypotenuse. Cosine is adjacent
over hypotenuse. Tangent is opposite
over adjacent. We can write that
the tangent of theta is equal to the length
of the opposite side-- 324 meters-- over the length
of the adjacent side-- over 54 meters. Now you might say,
well, OK, that's fine. What angle, when I take its
tangent, gives me 324/54? Well, for this, it will probably
be useful to use a calculator. And the way that
we'd use a calculator is we would use the
Inverse Tan Function. So we could rewrite
this as we're going to take the
inverse tangent-- and sometimes it's written as
tangent with this negative 1 superscript. So the inverse tangent
of tan of theta is going to be equal to the
inverse tangent of 324/54. And just to be clear, what
is this inverse tangent? This just literally
says, this will return what is the angle that,
when I take the tangent of it, gives me 324/54. This says, what is
the angle that, when I take the tangent of it,
gives me tangent of theta? So this right over here, this
just simplifies to theta. Theta is the angle that when
you get the tangent of it gets you tangent of theta. And so we get theta is equal
to inverse tangent of 324/54. Once again, this inverse tangent
thing you might find confusing. But all this is saying
is, over here, we're saying tangent of
some angle is 324/54. This is just saying
my angle is whatever angle I need so that when
I take the tangent of it, I get 324/54. It's how we will
solve for theta. So let's get our calculator out. And let's say that we want
our answer in degrees. Well, I'm just going
to assume that they want our answers in degrees. So let me make sure my
calculator is actually in degree mode. So I'll go to the 2nd
mode right over here. And actually it's in
radian mode right now. So let me make sure
I'm in degree mode to get my answer in degrees. Now let me exit out of here. And let me just type in
the inverse tangent-- so it's in this yellow
color right here-- inverse tangent of
324 divided by 54 is going to be-- and
they told us to round to two decimal
places-- 80.54 degrees. So theta is equal
to 80.54 degrees. That's the angle at which
you should shoot the gun to help defeat this
horrible alien.