Practice: Find The Perimeter Of A Triangle

Geometry Level 2

Let there be a triangle drawn on a Cartesian plane with vertices at coordinates ( 3 , 0 ) , ( 1 , 3 ) , ( 4 , 1 ) . (-3,0), (1,-3), (4,1). Find the perimeter of this triangle to the nearest integer.


The answer is 17.

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18 solutions

Raj Magesh
Dec 26, 2013

The distance between two points ( x 1 , y 1 ) \left(x_1, y_1\right) and ( x 2 , y 2 ) \left(x_2, y_2\right) in the Cartesian plane is given by ( x 2 x 1 ) 2 + ( y 2 y 1 ) 2 \sqrt{\left(x_2 - x_1\right)^{2} + \left(y_2-y_1\right)^{2}} .

Hence, the perimeter of the triangle is found by finding the distances between each pair of points and summing them:

Between ( 3 , 0 ) \left(-3, 0\right) and ( 1 , 3 ) \left(1, -3\right) :

D 1 = ( 1 ( 3 ) ) 2 + ( ( 3 ) 0 ) 2 D_1 = \sqrt{\left(1 - (-3)\right)^{2} + \left((-3) - 0\right)^{2}}

D 1 = 5 \Rightarrow D_1 = 5

Between ( 3 , 0 ) \left(-3, 0\right) and ( 4 , 1 ) \left(4, 1\right) :

D 2 = ( 4 ( 3 ) ) 2 + ( 1 0 ) 2 D_2 = \sqrt{\left(4 - (-3)\right)^{2} + \left(1 - 0\right)^{2}}

D 2 = 5 2 \Rightarrow D_2 = 5\sqrt{2}

Between ( 1 , 3 ) \left(1, -3\right) and ( 4 , 1 ) \left(4, 1\right) :

D 3 = ( 4 1 ) 2 + ( 1 ( 3 ) ) 2 D_3 = \sqrt{\left(4 - 1\right)^{2} + \left(1 - (-3)\right)^{2}}

D 3 = 5 \Rightarrow D_3 = 5

Hence, P e r i m e t e r = D 1 + D 2 + D 3 = 5 + 5 2 + 5 = 17.07... 17 Perimeter = D_1 + D_2 + D_3 = 5 + 5\sqrt{2} + 5 = 17.07... \approx \boxed{17}

Can I ask how you post with things under square root signs, with exponents, etc?

Dorian Thiessen - 7 years, 1 month ago

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Use LaTeX. When you type something here on Brilliant, there's a green link called the Formatting guide. Just click on it, and the thing that appears at the top will be the Easy Math Editor, which is a link to download a Chrome extension that allows you to edit math easily, I suppose. I haven't tried it yet; I use the inline LaTeX, which takes a bit of time to get used to.

Raj Magesh - 7 years, 1 month ago
Lim Zi Heng
Dec 26, 2013

Let point A A at coordinates ( 3 , 0 ) ( -3,0 )

Let point B B at coordinates ( 1 , 3 ) ( 1,-3 )

Let point C C at coordinates ( 4 , 1 ) ( 4,1 )

To measure the distance between two point, use:

( x 1 x 2 ) 2 + ( y 1 y 2 ) 2 \sqrt{(x_1-x_2)^2+(y_1-y_2)^2}

Line AB = ( 3 1 ) 2 + ( 0 3 ) 2 = 5 \sqrt{(-3-1)^2+(0-3)^2} = \boxed{5}

Line BC = ( 1 4 ) 2 + ( 3 1 ) 2 = 5 \sqrt{(1-4)^2+(-3-1)^2} = \boxed{5}

Line AC = ( 3 4 ) 2 + ( 0 1 ) 2 = 5 2 \sqrt{(-3-4)^2+(0-1)^2} =\boxed{5\sqrt{2} }

By using calculator, we get 5 2 = 7.07 5 \sqrt{2} = 7.07

P e r i m e t e r = 5 + 5 + 7.07 = 17.07 17 Perimeter = 5+5+7.07 =17.07 \Rightarrow \boxed{17}

So the answer is 17

Prasun Biswas
Dec 27, 2013

We shall use here the distance formula, d = ( x 2 x 1 ) 2 + ( y 2 y 1 ) 2 d=\sqrt{(x_{2}-x_{1})^{2}+(y_{2}-y_{1})^{2}}

The vertices are ( 3 , 0 ) , ( 1 , 3 ) , ( 4 , 1 ) (-3,0) , (1,-3) , (4,1) . Using the distance formula, we get,

1st side = ( 3 1 ) 2 + ( 0 + 3 ) 2 = ( 4 ) 2 + 3 2 = 16 + 9 = 25 = 5 =\sqrt{(-3-1)^{2}+(0+3)^{2}} = \sqrt{(-4)^{2}+3^{2}} = \sqrt{16+9} = \sqrt{25} = 5 units.

2nd side = ( 4 1 ) 2 + ( 1 + 3 ) 2 = 3 2 + 4 2 = 9 + 16 = 25 = 5 =\sqrt{(4-1)^{2}+(1+3)^{2}} = \sqrt{3^{2}+4^{2}} = \sqrt{9+16} = \sqrt{25} = 5 units.

3rd side = ( 4 + 3 ) 2 + ( 1 0 ) 2 = 7 2 + 1 2 = 49 + 1 = 50 = 5 2 =\sqrt{(4+3)^{2}+(1-0)^{2}} = \sqrt{7^{2}+1^{2}} = \sqrt{49+1} = \sqrt{50} = 5\sqrt{2} units.

Now, Perimeter = 5 + 5 + 5 2 = 10 + 5 × 1.414 = 10 + 7.07 = 17.07 =5+5+5\sqrt{2}=10+5\times 1.414 = 10+7.07 = \boxed{17.07}

So, perimeter of the triangle to the nearest unit is 17 \boxed{17} units.

Sanghamitra Anand
Dec 28, 2013

The point (-3, 0) lies in the 2nd quadrant.
The point (1, -3) lies in the 4th quadrant.
The point (4, 1) lies in the 3rd quadrant. Always the 'x' coordinate will be given at the first in a set of coordinates.
Consider any two sets (since, two points form a line i.e. the side of the triangle)
I've taken the first set as (-3, 0) and (1, -3), the second as (1, -3) and (4, 1), and the third as ((-3, 0) and (4, 1)
Let the 'x' and 'y' coordinates of the first set that you are considering be 'x1' and 'y1' and the coordinates of the second set be 'x2' and 'y2'
Use the formula to calculate the distance between the points:
√(x2 - x1)² + (y2 - y1)²
Substitute the values in the formula:
The distance between the first two points = √(1 + 3)² + (-3 - 0)²
= √4² + (-3)²
= √16 + 9
= √25 = 5
The distance between the next two points = √(4 - 1)² + (1 + 3)²
= √3² + 4²
= √9 + 16
= √25 = 5
The distance between the next set of points = √(4 + 3)² + (1 - 0)²
= √7² + 1²
= √49 + 1
= √50 = 7.07 ≈ 7
The distances are the lengths. Therefore, the perimeter of the triangle = 5 + 5 + 7 = 17





the answer is 17.07 units

Ankita Chowdhury - 7 years, 2 months ago

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17 is the nearest approximated unit to 17.07!

Sanghamitra Anand - 7 years, 2 months ago
Budi Utomo
Dec 24, 2013

Assumt that it triangle ABC, first to side AB is sqrt((3-(-1))^2 + (0-(-3))^2 )= 5. Second, side BC = sqrt((1-(-3)^2 + (4-1)^2) = 5. Finally, side AC = sqrt((4-(-3)^2 + (1-0)^2) = 5 * sqrt(2) = 5 * 1,4 = 7. ---> Thus, the perimeter of this plane triangle is 5 + 5 + 7 = 17.(Answered)

Good job. That was my method. Next time, though, try to use LaTeX

Sharky Kesa - 7 years, 5 months ago

i did it but i get the perimeter of the triangle is 5 + 6 + 7 = 18

Hima Nasr - 7 years, 5 months ago

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How did you arrive at 6 6 ??

Happy Melodies - 7 years, 5 months ago

its 5+5+7

Ephraim Raj - 7 years, 3 months ago
Dorian Thiessen
Apr 24, 2014

I thought it said area when I read it through the first time, no wonder I kept getting it wrong. I thought I got it wrong when I did (1/2)(side 1)(side 2) so I tried (1/2)(base)(height) which took awhile cause I had to get the formula for two of the lines, then the formula for the line perpendicular to the base that went through the tip (the bottom point). Gah.

The area rounds to 13 units squared if anyone is curious :P

Haha! Well, if you want, there's a far quicker method for finding the area of a triangle (or indeed, any polygon, AFAIK), in a Cartesian plane. It's called the Shoelace formula and for a triangle, it's basically this:

A = 1 2 × [ x 1 ( y 2 y 3 ) + x 2 ( y 3 y 1 ) + x 3 ( y 1 y 2 ) ] = 1 2 × 25 = 12.5 13 A = \dfrac{1}{2} \times [x_1(y_2-y_3) + x_2(y_3-y_1) + x_3(y_1-y_2)] = \dfrac{1}{2} \times 25 = 12.5 \approx \boxed{13}

Raj Magesh - 7 years, 1 month ago
John Ray Matugas
Mar 27, 2014

Using the Distance Formula get the sides of the triangle then add for the Perimeter :)

Ephraim Raj
Mar 2, 2014

easiest way- use the Pythagoras theorem

Yes! a^2 +b^2 = c^2 is the easiest way. I used the axes to generate right angle triangles with the perimeter sides as hypotenuses.
So the first triangle has 1 unit height, 7 unit length so its hypotenuse is sqrt(50) or 7.
Second and third triangle have a base 4 height 3 so H=5.

Maddy Feltus - 2 years, 11 months ago
Steve Rogers
Feb 17, 2014

Perimeter = sum of all the sides = 5 + 5 + sqrt(50) = 17 approximately

Lalit Pathak
Jan 29, 2014

Distance b/w any two points can be found using the following formula from coordinate geometry :

\sqrt{(x1-x2)^{2}+(y2-y1)^{2}+(z2-z1)^{2}}

Using the above formula we find \sqrt{50} ,5 and 5, so perimeter which is the measure of the sum of the sides of tehe triangle so we get 17(around).

Henry Okafor
Jan 17, 2014

The method known is the find the sides of the triangle by using the formula to measure the distance between two points.

Hence, with that method, the first side is 1 + 49 \sqrt{1 + 49} = 50 \sqrt{50} = 12.07 Also, the other sides are 5 each. Hence the perimeter of the triangle is 5 +5 + 12.07 = 17.07

Soubhik Paul
Jan 3, 2014

Distance between (-3,0) and (1,3) is 5; [sqrt(16+9)]; similarly other two sides are 5 and 5*1.732=7; So Perimeter is 5+5+7=17

Rohit Nair
Dec 27, 2013

Let, A = (-3,0) B = (1,-3) C = (4,1) AB = ((-3-1)^2 + (-3-0)^2)^1/2 = 25^1/2 = 5 BC = ((4-1)^2+(1--3)^2)^1/2 = 25^1/2 = 5 AC = ((-3-4)^2 +(1-0)^2)^1/2 = 50^1/2 Perimeter of triangle ABC = AB + BC+ AC = 5 + 5 + 7.07 = 17.07 = 17(Rounded Off)

By distance formula, get 5 + 5 + 50 5 + 5 + \sqrt{50} . We know 7 < 50 < 8 7 < \sqrt{50} < 8 . Thus, the answer is 5 + 5 + 7 = 17 5 + 5 + 7 = \boxed {17} .

Rajat Khandelwal
Dec 26, 2013

Let A = (-3,0) , B = (1,-3) , C = (4,1) By distance formula we get AB = 5, BC = 5 and AC = root 50 Perimeter = AB+BC+AC = 5+5+root50 = 17.02 units.

Tommy Räjert
Dec 26, 2013

I wrote it down on a piece of paper, marked off the dots and drew the triangle(x,y). Then I used the pythagoran theorem to conclude that the upper line((-3,0)-(4,1)) were about 7 units(-3to4=7,0to1=1 sqrt(1^2+7^2)). Then I did the same on the other two and they were =5(the beautiful equation sqrt(3^2+4^2)=5). 7+5+5=17. DONE!

Divyansh Singhal
Dec 26, 2013

Use distance formula between two co-ordinates.

Ashutosh Krishna
Dec 26, 2013

let A(-3,O) B(1,-3) C(4,1) THEN AB=5 UNITS

BC=5 UNITS

AC=7.07(APPROX BY UNDER ROOT 50)

AB+BC+AC=5+5+7.07

                   =17.07


                  =17 UNITS(NEAREST UNIT)

Apply distance formula b/w 2 coordinates

farah khalid - 7 years, 5 months ago

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