Large Sides Means Large Area, Right?

Geometry Level 3

Which of the following triangles has a larger area:

  • triangle A with side lengths 13 , 13 , 10 13, 13, 10 , or
  • triangle B with side lengths 13 , 13 , 24 ? 13, 13, 24\, ?
Triangle A Triangle B Their areas are equal Cannot be determined

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19 solutions

Pooja Nath
Jan 15, 2016

Both the triangles are isosceles triangles, which means that median of the triangle on the non-equal side is also the normal. We have two pictures now:

Thus both the triangles have the same area now, which 12*5 = 60 sq units.

Nice! The diagram makes it really obvious :)

Calvin Lin Staff - 5 years, 5 months ago

Wow, I did it with Heron's formula and took forever. It goes to show that drawing diagrams should be the first step.

A Former Brilliant Member - 4 years, 3 months ago

Of course that's from where the numbers were similar you just needed to know that 5^2 + 12^2 = 13^2, beautiful solution.

Liviu Vigu-Giurea - 3 years, 9 months ago

I did it the same way, but I wonder if you or someone would be good enough to tell me how to include diagrams in a solution that I post (which would be very helpful in a solution of the side-length of a square that I posted, though without a diagram). Regards, David

David Fairer - 3 years, 6 months ago

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On desktop, the is a toolbar when you're submitting a solution. Clicking on the third icon from the left allows you to upload an image to be displayed.

Note: We use markdown formatting, so if you already have the image uploaded, you can just type in ![](URL) .

Calvin Lin Staff - 3 years, 6 months ago

I don't like this problem statement much. The fact that it doesn't mention what units of length made me figure this was indeterminate. i.e. one could claim the first triangle was inches, the second in cms or some such. Tony

Tony Miller - 3 years, 2 months ago

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If units are not specified, then the (unwritten) assumption is that they are in the same units.

Calvin Lin Staff - 3 years, 2 months ago

Wow it take me so long to get the hag of this

Tigiste Asefa - 2 years, 4 months ago

actually, it should be 5*12 divided by 2, which is 30

Alan Li - 1 year, 11 months ago

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Remember that each original triangle comprises 2 of these 5-12-13 right triangles, which is why the formula wasn't divided by 2.

Calvin Lin Staff - 1 year, 11 months ago

"S" means "semi-perimeter", not "squared area" or something, as it may look.

Dead Boy - 11 months, 2 weeks ago
Saurabh Mallik
May 18, 2014

By Heron's formula:

Triangle A:

S 1 = 13 + 13 + 10 2 = 36 2 = 18 S_{1}=\frac{13+13+10}{2}=\frac{36}{2}=18

Area ( A 1 ) = S ( S a ) ( S b ) ( S c ) = 18 ( 18 13 ) ( 18 13 ) ( 18 10 ) (A_{1})=\sqrt{S(S-a)(S-b)(S-c)}=\sqrt{18(18-13)(18-13)(18-10)}

= 18 × 5 × 5 × 8 =\sqrt{18\times5\times5\times8}

= 2 × 3 2 × 5 2 × 2 3 =\sqrt{2\times3^{2}\times5^{2}\times2^{3}}

= 2 4 × 3 2 × 5 2 =\sqrt{2^{4}\times3^{2}\times5^{2}}

= 2 2 × 3 × 5 = 4 × 15 = 60 =2^{2}\times3\times5=4\times15=60

Triangle B:

S 2 = 13 + 13 + 24 2 = 50 2 = 25 S_{2}=\frac{13+13+24}{2}=\frac{50}{2}=25

Area ( A 2 ) = S ( S a ) ( S b ) ( S c ) = 25 ( 25 13 ) ( 25 13 ) ( 25 24 ) (A_{2})=\sqrt{S(S-a)(S-b)(S-c)}=\sqrt{25(25-13)(25-13)(25-24)}

= 25 × 12 × 12 × 1 =\sqrt{25\times12\times12\times1}

= 5 2 × 1 2 2 × 1 2 =\sqrt{5^{2}\times12^{2}\times1^{2}}

= 5 × 12 × 1 = 60 =5\times12\times1=60

A 1 = A 2 = 60 A_{1}=A_{2}=60

Thus, the answer is E q u a l \boxed{Equal}

Yeah I use the same method with you, it is easy!

Tấn Phát Nguyễn - 4 years, 6 months ago

Exactly the way I have done! It is the best way

Syed Hamza Khalid - 4 years, 1 month ago

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Hardly the best way.

Rudrayan Kundu - 2 years, 5 months ago

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Yeah come on that was me 2 years ago; (I think it was when I started learning to solve problems in a beautiful way) :)

Syed Hamza Khalid - 2 years, 5 months ago
Mardokay Mosazghi
May 13, 2014

The first method is easy the triangles are both right triangles with side lengths (5,12,13).But if you didn't realize that First find the length of the height and then you are going to realize that the base and the height are the same for the two triangles proving that the area would be same.The length and base should be be 12 and 5 in both triangles. In advance you could use herons formula

I click wrong lol...but using logical thinking it's equal....no need calculation at all....

Zack Yeung - 5 years, 10 months ago

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hello, can u explain it? ..I used Heron's formula, but logical way is most interesting, so please. :)

Rosta Plachý - 5 years, 10 months ago

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I don't know what is herons' formula haha....triangle's formula is 1/2 times base times height ....you draw it out or imagine it using 13 as base and height....sub into formula it would be just the same.....because hypothenuse isnt the main point in the formula i guess...

Zack Yeung - 5 years, 10 months ago

no it is correct

Megha Malyala - 5 years, 6 months ago

using heron's formula it would be simple

Saurav Sharma - 7 years, 1 month ago

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I disagree. This method is obviously faster, more elegant and relies on less calculation.

Mursalin Habib - 7 years, 1 month ago

how it is? please explain it briefly

sri nath - 7 years ago

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Heron's formula: Area of a triangle = under root of s(s - a) (s - b) (s - c)
where S is half the perimeter of the triangle and a, b, c are sides of the triangle. Use this on both the triangles and you'll see that both areas are equal to 60

Deeksha Das - 6 years, 11 months ago

You can refer my solution.

Saurabh Mallik - 7 years ago

This solution by Mardokay is good. Only, it doesn't always strike us that we should probably see if the sides happen to be a pythagorean triplet. Heron's formula strikes first.

Deeksha Das - 6 years, 11 months ago

Dont understand fully. As far as i know, the area of triangle is 0.5 x width x height.

And normally the numbers given are l x w x h ....so height would be 10 and 24 and therefore not the same area ?

What am i missing/overseeing in this assumption ?

Thanks, Bent

Bb B - 3 years, 3 months ago

But how can you figure out the height?

Alexander Brown - 2 years, 8 months ago
Michael Mendrin
May 13, 2014

Check out the 5:12:13 right triangle in both cases.

Michael Mendrin : Yours is by far the easiest and shortest method. Pooja Nath's solution - by using diagrams - explains where the 5:12:13 triangles come from.

John Conway - 5 years, 4 months ago

Thumbs up!

Eddie Sneeh - 2 years, 9 months ago
Lew Sterling Jr
Jul 10, 2015

You should send your peers over to Brilliant so that they can learn from the community!

Calvin Lin Staff - 5 years, 11 months ago

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I sent some already on here and others...well...if math was a person, they would run away from it.

Lew Sterling Jr - 5 years, 11 months ago

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I promise I won't bite.

Yea I understand. But that's cos they haven't done been on Brilliant :)

Calvin Lin Staff - 5 years, 11 months ago

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@Calvin Lin I know you don't bite. A few of my peers did practice from my EOC sets that I did. I will be finishing the Geometry EOC set though since I have gotten some notices from people to finish it.

Lew Sterling Jr - 5 years, 11 months ago

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@Lew Sterling Jr That's really exciting!

Calvin Lin Staff - 5 years, 11 months ago

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@Calvin Lin Yeah. I have about 30-40 questions I got to do for the Geometry EOC set.

Lew Sterling Jr - 5 years, 11 months ago

I cant picture it because the with is 24, much wider than 10. since they have two 13 side i can picture out that they are not equal. so cool.

Klyvan Pajarillo - 5 years, 6 months ago

isn't that a semiperimeter not a perimeter ?

Dan Petrache - 5 years, 6 months ago
Vipul Sourav
May 13, 2014

Very easy use herons formula

Triangle A. (Semi perimeter=18) area=√18(18-13)(18-13)(18-10) =√18 5 5*8 =√3600 =60

Triangle B (semi perimeter =25) Area=√25(25-13)(25-13)(25-24) =√25 12 12*1 =√3600 =60

Asad Ullah
May 13, 2014
  1. draw a construction line perp from mid of the base of the triangles,
  2. find the height of a triangle and simply
  3. then apply formula of area of triangle, as A= (Height x Base)/2
    for both the triangle the procedure used is same and then
  4. campare the both Areas of a Traingle
Sudarshan Pathi
Nov 25, 2015

The height of an isosceles triangle can be calculated by using pythagoras theorem. Consider the unequal side as the base. One of the equal sides and half of the length of the unequal side and the height form a right triangle. The equal side serves as the hypotenuse.

Hence, Square(Equal Side)= Square(Height) + Square(Half of Base or Unequal Side)

Thus, for each of the triangles, Height = 12 , Base = 10 and area = 60 (Triangle 1) [square(13)=square(12-height)+square(5-half of base)] Height = 5, Base=24 and area=60 (Triangle 2).[square(13)=square(5-height)+square(12-half of base)]

Therefore the triangles have equal area.

Great! Finding the height makes it simple to find the area of the triangle.

Calvin Lin Staff - 5 years, 6 months ago
Guiseppi Butel
Jul 11, 2014

In an isosceles triangle the altitude bisects the dissimilar base.

Use c^2 = a^2 + b^2 to solve for the altitudes.

Then find the areas. They both equal 60.

only one property of an isosceles triangle can make this just a two lines sum........that is the perpendicular when dropped on the base divides the base into two equal parts.............in the first triangle.half of the base is 5 and its ht is 12.............in second triangle.half of the base is 12 and its ht is 5............now area of a triange is 1/2.base .altitude value of whose is same for both triangles ........thats it...........!!!!!!!!

Alkis Piskas
Oct 21, 2018

In the figure below, the vertical to the base of the isosceles triangle, h = sqrt(a^2-(b/2)^2). The area of the triangle is (h * b) / 2 = (sqrt(a^2-(b/2)^2) * b) / 2. Replacing a with 13 and b with either 10 or 24, the result is always 60.

Alex De Martino
Feb 27, 2018

Think about Pythagorean triples! (5,12,13) is a triple so it immediately gives the answer.

I want calculus language.

Am Kemplin - 1 month, 2 weeks ago
Whitney Clark
Nov 26, 2015

Bisect them and you get two triangles, 5-12-13 on the sides.

Both are isosceles triangle. For triangle A find height =sqrt(13^2 - 5^2) =12, so area of triangle 'A'= 1/2 (base)(height)=1/2 (10)(12)=60; for triangle 'B' height= sqrt(13^2 - 12^2)=5, hence area of 'B'=1/2(24)(5)=60.

Deeksha Das
Jul 6, 2014

Use heron's formula and find out

Chandan Sharma
Jun 3, 2014

Use Heron's Formula. Most Mathematical to Prove!!

Arshad R Shaikh
May 29, 2014

it is very is problem with some calculation by determine area using heron's formula. s=\frac{a+b+c}{2}. A = \sqrt{s(s-a)(s-b)(s-c)},

Heron's Formula

Finn Hulse
May 13, 2014

Bashed it out with Heron's. I'm not sure exactly why, but somehow they are equal. :D

Take two 5-12-13 triangles. Put them together on the 5 face.

Take two more 5-12-13 triangles. Put them together on the 12 face.

Do you see now?

Daniel Liu - 7 years, 1 month ago

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HOLY COW. That's why, huh. Okay, I get it. :D

Finn Hulse - 7 years, 1 month ago

Another [cryptic] hint on why they are equal sin θ = sin ( π θ ) \sin \theta= \sin (\pi-\theta) .

Mursalin Habib - 7 years, 1 month ago

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