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## High school geometry

### Course: High school geometry > Unit 6

Lesson 3: Problem solving with distance on the coordinate plane- Area of trapezoid on the coordinate plane
- Area & perimeter on the coordinate plane
- Points inside/outside/on a circle
- Points inside/outside/on a circle
- Challenge problem: Points on two circles
- Coordinate plane word problem
- Coordinate plane word problems: polygons

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# Area of trapezoid on the coordinate plane

Sal finds the area of a trapezoid using the distance formula and the trapezoid area formula.

## Want to join the conversation?

- I do not know how to do all the shapes that you give on the the exercise(17 votes)
- Why are you using things like "6 square roots of 5" when you could just put the square root of 180 in a calculator and be done with it? It seems to me like you're just adding a bunch of steps and making it far more complicated than it needs to be.(5 votes)
- The one issue is the difference between an exact answer and an rounded answer, 6 √5 is the exact answer simplified, using a calculator will require you to round.(13 votes)

- My geometry teacher says that in order for the height to be a height in a trapezoid it needs to be perpendicular to one of the bases. Is this true for ALL trapezoids? And sorry I ask a lot of questions.(5 votes)
- Height is always perpendicular to the base, that is true for all figures, triangles, rectangles, parallelograms, and trapezoids(9 votes)

- I'm in sixth grade math and I don't know how to do what you did at4:29.(4 votes)
- That was just another way of doing the Pythagorean Theorem to find the longest side of a right triangle.

a^2 + b^2 = c^2, where "c" is the longest side of the triangle.

Since you have to find the square root anyway, you can put the "a^2 + b^2" in a square root and immediately solve for "c".

So all Sal did here was do two steps at once.(2 votes)

- I worked through a perimeter problem which at the end had me with 8 + √89 + √89 + √32. I added all the square roots together and got:

8 + √89 + 89 + 32

= 8 + √210

They wanted a decimal to the tenth so I finished with ≈22.5.

However, that was wrong instead they did took the square roots before adding:

8 + √89 + √89 + √32

≈ 8 + 9.43 + 9.43 + 5.66

And got ≈32.5.

So obviously these come out with two vastly different answers. So what are the rules here why is this approach the correct one?

Thanks!(3 votes)- You can't add the contents of square roots. If you could, then √(9)+√(16) would equal √(9+16)

√(9)+√(16) = 3+4 = 7

√(9+16) = √(25) = 5

Hopefully you can see that adding the contents of radicals doesn't work. This is why they did the square roots first. Since each of the square roots creates an irrational number, the results were rounded.

Hope this helps.(3 votes)

- So you guys don't have any explanation on perimeters. Can you explane how to find the Perimeter of a Trapizoid on a coordinate plane?(2 votes)
- once you've found the lengths of all the sides using the Pythagoras theorem, instead of plugging those values into the area formula, simply add them up.(4 votes)

- Any videos on Khan Academy that explains how to calculate for example √9*5 into ∛5? Or maybe someone can explain the logic. It seems that the 3 before the √ came from the fact that it is the square root of 9. Yet I don't get equivalent answers from the following equations, which would be applying my logic: √4*8 and 2√8, so I must be missing something.(2 votes)
- So the first issue is that you show the cubed root rather than 3√5 which is different. The logic is that √(9*5) can be separated to √9*√5, and since √9 = 3, you get 3√5. The second issue is that your problem is not completely simplified. So √(4*8) (notice I keep adding parentheses) can be shown as √(4*4*2) = √4*√4*√2 = 2*2*√2=4√2. I am not sure what you mean by not getting equivalent answers except for the fact that you did not break 8 down into a perfect square * a non-perfect square.(3 votes)

- Ok that makes sense but i still dont understand how to find the piermeter on a coordinate plane with points, idk why but something is just not clicking in my brain(3 votes)
- Well, you just need to find the distance between two points. You can simply use Distance formula or Pythagorean theorem.(1 vote)

- on the change of y of the height, how did he get -4?(3 votes)
- Because the line was sloping down. If you ever learned slope then this concept of negative change should be simple(1 vote)

- How do you do everything?(3 votes)

## Video transcript

- [Voiceover] So we have a trapezoid here on the coordinate plane
and what we want to do is find the area of this trapezoid,
just given this diagram. And, like always, pause this video and see if you can figure it out. Well, we know how to figure
out the area of a trapezoid. We have videos where
we derive this formula. But, the area of a
trapezoid, just put simply, is equal to the average of
the lengths of the bases, we could say base one plus
base two times the height. And, so what are our bases here? What are, and what is going
to be our height over here? Well, we could call base one, we could call that segment CL, so it would be the length of
segment CL, right over here. I'll do that in magenta. That is going to be base one. Base two, base two, that could, we'll do that in a different color. Base two would be the
length of segment OW, or b two would be the length of segment OW, right over there. And then, our height, our height h, well, that would just be an altitude and they did one in a dotted line, here. Notice it intersects the, base one, I guess you could say segment
CL at a right angle, here. And so, this would be,
this would be the height. So, if we know the
lengths of each of these, if we know each of these values, which are the lengths of these segments, then we can evaluate the area
of this actual trapezoid. And, once again, if this is
completely unfamiliar to you or of you're curious,
we have multiple videos talking about the proofs or how we came up with this formula. You an even break out,
break down a trapezoid into two triangles and a rectangle, which is one way to think about it. Anyway, let's see how we
could figure this out. So, the first one is,
what is b one going to be? B one is the length of segment CL and you could say, "We'll
look, we know what these, "the coordinates of these points are." You could say, "Let's use
the distance formula," and you could say, "Well,
the distance formula is just "an application of the
Pythagorean Theorem." So, this is going to be the square root of our
change in x squared, so our change in x is going
to be this, right over here. And, notice, we're going
from x equals negative four to x equals eight, as w go from C to L. So, our change in x is equal
to eight minus negative four which is equal to 12. And, our change in in y, our change in y, we are going from y equals
negative one to y equals five. So, we could say our change in y equal to five minus negative one which, of course, is equal to six. And, you see that: one,
two, three, four, five, six. And, the segment that we care about, its length that we care about,
that's just the hypotenuse of this right triangle
that has one side 12 and one side six. So, the length of that hypotenuse, from the Pythagorean Theorem, and, as I mentioned,
the distance formula is just an application of
the Pythagorean Theorem, this is going to be our change in x, squared, 12 squared, plus our change in y,
squared, so plus six squared, and this is going to be
equal to 144 plus 36. So, the square root of 144 plus 36 is one hundred, one hundred and 80 which is equal to, let's
see, 180 is 36 times five. So, that is six square roots,
six square roots of five. Oh, let me not skip some steps. So, this is the square
root of 36 times five which is equal to, square root of 36 is 6, so six square roots of five. Now, let's figure out b two. So, b two, once again, change in x squared plus the square root
of change in x squared plus change in y squared. Well, let's see, if we're going from, we could set up a right
triangle, if you like, like this, to figure those things out. So, our change in x, we're
going from x is at negative two, x is going from negative
two to positive four. So, our change in x is six. Our change in y, we are going from, we are going from y equals, y
equals five to y equals eight. So, our change in y is equal to three. So, just applying the Pythagorean Theorem to find the length of
the hypotenuse, here, is going to be the square root of change in x squared, six squared, plus change in y squared,
plus three squared which is going to be equal to, it's going to be equal to 36 plus nine, which is 45, so, square root of 45 which is equal to the square
root of nine times five which is equal to three
square roots of five. And so, we only have
one left to figure out. We have to figure out h. We have to figure out the length of h. So, h is going to be equal to, and so, what is our, if
we're going from W to N, our change in x is two. Change in x is equal to two. We're going from X equals
four to x equals six. If you want to do that purely numerically, we would say, "Okay, our
endpoint, our x value is six. "Our starting point, our x-value is four. "Six minus four is two." You see that visually, here. So, it's going to be the square root of two squared plus our, let me write that radical
a little bit better. So, it's the square root
of our change in x squared plus our change in y squared. Our change in y is negative four. Change in y is negative four but we're going to square it, so it's going to become a positive 16. So, this is going to be equal to the square root of four plus 16. Square root of 20, which is equal to the square
root of four times five, which is equal to two times
the square root of five. It's nice that the square
root of five keeps popping up. And so, now we just substitute into our original expression. And so, our Area of our
trapezoid is going to be one half times six square roots of five, six square roots of five, plus three square roots of five, plus three square roots of five, let me close that parentheses, times two square roots of five, times two square roots of five. And, let's see how we can simplify this. So, six square roots of five plus three square roots of five, that is nine square roots of five. Let's see, the one half times the two, those cancel out to just be one. And so, we're left with nine square roots of five
times the square root of five. Well, the square root of five
times the square root of five is just going to be five. So, this is equal to nine times five which is equal to 45 square
units or units squared.