Inside perimeters

Geometry Level 1

A rectangle is divided into nine sub-rectangles, as shown in the figure. Inside some rectangles are their respective perimeters .

What is the perimeter of the large, outer rectangle?

28 36 56 72

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13 solutions

Venkatachalam J
Nov 5, 2017

Wow, I use another method, this is clever! Or 12+6+6+8+4-4-4 ? demonstrated by the graph?

Kelvin Hong - 3 years, 7 months ago

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No need to remove 4 two times: 4 is already a sum of the four sides. Really clever, I didn't think of doing it this way neither!

Steve Bergeron - 3 years, 7 months ago

I didn't even think of doing it that way! Nicely illustrated. Thank you.

Emma Boonzaier - 3 years, 7 months ago

That is nice! I went like this - if the sides are all integers, the central quadrilateral has to be a square with side length 1 (since 2 of its sides will have total length 2, and this can only be true if both have a length of 1). Then it is easy to calculate the side lengths for all the other sub-rectangles as well. You end up with length of the big rectangle = 5+1+2, and width = 2+1+3. Thus the perimeter is 14 x 2 = 28. Not a particularly elegant solution, but a simple one.

Swati Singh - 3 years, 7 months ago

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This is how I did it as well. the centre rectangle has to be square, with each side having length equals 1. You can then infer what the dimensions of the other coloured rectangles have to be since the edge they share with the centre rectangle has to be equal to 1.

James Cheetham - 3 years, 6 months ago

need to find only the semi périmètre of All four black rectangle and add the center blues rectangle perimeter: left high black semi perimeter is ((12+6)-4)/2 = 7 + left low black semi perimeter is ((12+8)-4)/2. = 8 + right higth black semi perimeter is ((6+6)-4)/2 = 4 + right high black semi perimeter is ((6+8)-4)/2 = 5 + blue périmètre = 4 =28 ;)

CREACH ALAIN - 3 years, 7 months ago

I tried to eyeball the measurement. I think drawing a diagram works best. Next time I should try to actually draw it again. I would rate this problem a 2 because I already know how perimeter works. I wanted to try eyeballing it. But I think it's better to redraw it.

Lucia Tiberio - 3 years, 7 months ago

That took me several minutes to understand, but once I did, I realized how beautifully elegant your answer was. Bravo!

Rex Henderson - 3 years, 7 months ago

it's so easy to find the solution, when you think that the "4" rectangle is a square, but to accept the solution first you should explain why the "4" rectangle is a square.

Hapi Pare - 3 years, 6 months ago

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the "4" rectangle is not necessarily a square.

Samuel Li - 3 years, 5 months ago

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No , the "4" rectangle will be a square , perimeter of a rectangle = 2(l + b)

2(l + b) = 4

l + b = 2

So, both l and b will be 1 because their sum add up to 2 . So now opposites sides in a rectangle are equal so all the four sides are equal to 1 and therefore it is a square .

Ram Mohith - 3 years, 2 months ago

The perimeter of the large, outer triangle (see the figure on the right) is composed of 3 three red lines, three green lines, three orange lines and three violet lines. The total perimeter of the three smaller rectangles (see the figure on the left;exclude the blue rectangle) is 12+6+6+8=32 and it is composed of four red lines, four green lines, four orange lines and four violet lines. So to get the perimeter of the biggest rectangle we need to subtract one red line, one green line, one orange line and one violet line. And that is exactly the perimeter of the blue rectangle (see left figure). Therefore, the desired answer is 32-4=28.

Nicely explained! Thank you.

Emma Boonzaier - 3 years, 7 months ago
Adriano Arce
Nov 5, 2017

Notice that we can identify each green/red/orange/purple side with a black side, so the perimeters of the green/red/orange/purple rectangles cover the perimeter of the black rectangle. However, there are 4 colored sides left over - but they each can be identified with a side of the blue rectangle. So the black rectangle's perimeter is: ( 6 + 12 + 6 + 8 ) 4 = 28 (6 + 12 + 6 + 8) - 4 = \boxed{28}

Simpler to assume that the central box is 1 x 1 and calculate all others. e.g. the 12 box must be 1 x 5, the 8 box is 1 x 3, etc.

Hilton Weiss - 3 years, 6 months ago

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Yes, but that is not necessarily true. It is possible that the dimensions of the inner blue shape are 1.5 and 0.5, not 1 and 1.

John Layton - 3 years, 6 months ago

Let the side lengths be assigned as a a , b b , c c , d d , e e and f f as shown in the figure. Then the parameter of the the larger, outer rectangle is given by p = 2 ( a + b + c + d + e + f ) p = 2(a+b+c+d+e+f) and we have:

{ 2 a + 2 e = 12 . . . ( 1 ) 2 b + 2 d = 6 . . . ( 2 ) 2 b + 2 e = 4 . . . ( 3 ) 2 b + 2 f = 8 . . . ( 4 ) 2 c + 2 e = 6 . . . ( 5 ) \begin{cases} 2a+2e = 12 & ...(1) \\ 2b+2d = 6 & ...(2) \\ 2b+2e = 4 & ...(3) \\ 2b+2f = 8 & ...(4) \\ 2c+2e = 6 & ...(5) \end{cases}

Adding ( 1 ) + ( 2 ) + ( 3 ) + ( 4 ) + ( 5 ) (1)+(2)+(3)+(4)+(5) , we have:

2 a + 6 b + 2 c + 2 d + 6 e + 2 f = 12 + 6 + 4 + 8 + 6 = 36 2 a + 2 b + 2 c + 2 d + 2 e + 2 f + 4 b + 4 e 36 2 ( a + b + c + d + e + f ) = 36 2 ( 2 b + 2 e ) ( 3 ) : 2 b + 2 e = 4 p = 36 2 ( 4 ) = 28 \begin{aligned} 2a+6b+2c+2d+6e+2f & = 12+6+4+8+6 = 36 \\ 2a+2b+2c+2d+2e+2f + \color{#3D99F6} 4b+4e & - 36 \\ 2(a+b+c+d+e+f) & = 36 - 2\color{#3D99F6}(2b+2e) & \small \color{#3D99F6} (3): \ 2b+2e = 4 \\ p & = 36 - 2{\color{#3D99F6}(4)} = \boxed{28} \end{aligned}

Nice job. I like your relatively rigorous method/explanation best (you accidently forgot an "= 0" in the 3rd to last line). I was headed down the same path when I realized the perimeter of the cross formed by the 5 given tiles is equivalent to the perimeter of the outer rectangle, similar to other methods posted. It is apparent that the perimeter of the cross formed by the 5 given tiles is the perimeter of each outer tile (12,6,6,8) minus the center tile (4).

Keith V - 3 years, 7 months ago

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Thanks, but @Venkatachalam J 's solution is better.

Chew-Seong Cheong - 3 years, 7 months ago

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I wouldn't say better or worse, just different. Most people saw the "trick" geometrically, you algebraically.

Keith V - 3 years, 7 months ago

I try something like that at first too, but I was too lazy to go that path! :D Nicely done sir!

Steve Bergeron - 3 years, 7 months ago
Jay Zhong
Nov 6, 2017

Dumbest, yet the fastest method: This response is based on an assumption, which is that the rectangle with a perimeter of 4 has side lengths of 1, therefore making it a square. Hence, you can easily find the side length and width of the larger rectangle being 8 and 6 respectively. Add them up and multiply by 2, and you'll get 28.

Maybe the dumbest, but that’s what I did.

Using mental arithmetic too!

Ananda Woodley - 3 years, 7 months ago

Like Ananda says, it is the dumbest and technically wrong unless you check solution, but of course it is what I did as well. Upvote for common answer

Adrian Self - 3 years, 7 months ago

I find issue with the fact that the box with a perimeter or 8 is smaller than the ones with 6...

- Silence - - 3 years, 7 months ago

Me too, I know if was faulty solution but still celebrated the “win”. I am interested to know when this faux solution would fail to get the right answer. Is it to do with the internal perimeters, vertical and horizontal conveniently being nice easy multiples of each other?

Nelson Bibby - 3 years, 5 months ago
Zain Majumder
Nov 5, 2017

Two side lengths of a rectangle add to half the perimeter of that rectangle, so:

B + D = 6 ÷ 2 = 3 B+D=6\div2=3

A + E = 12 ÷ 2 = 6 A+E=12\div2=6

C + E = 6 ÷ 2 = 3 C+E=6\div2=3

B + F = 8 ÷ 2 = 4 B+F=8\div2=4

Add these up to get A + 2 B + C + D + 2 E + F = 16 A+2B+C+D+2E+F=16 .

Also, note that the two blue rectangle side lengths are B B and E E .

B + E = 4 ÷ 2 = 2 B+E=4\div2=2

Subtract this new equation from the last to get:

A + B + C + D + E + F = 14 A+B+C+D+E+F=14

The total perimeter is 2 × 14 = 28 . 2\times14=\boxed{28}.

Luis Salazar
Nov 9, 2017

how is it?

rizuanul alam - 3 years, 7 months ago
Emmanuel David
Nov 7, 2017

So we are looking for 2 ( x + y + z ) + 2 ( a + b + c ) 2(x + y + z) + 2(a + b + c) which is also equivalent to 2 a + 2 b + 2 c + 2 x + 2 y + 2 z 2a + 2b + 2c + 2x + 2y + 2z

2 y + 2 a = 12 2y + 2a = 12

2 x + 2 b = 6 2x + 2b = 6

2 y + 2 b = 4 4 y + 4 b = 8 2y + 2b = 4 \Rightarrow 4y + 4b = 8

2 z + 2 b = 8 2z + 2b = 8

2 c + 2 y = 6 2c + 2y = 6

Adding all the equations, we get:

2 a + 6 b + 2 c + 2 x + 6 y + 2 z = 36 2a + 6b + 2c + 2x +6y + 2z = 36

And we are looking for 2 a + 2 b + 2 c + 2 x + 2 y + 2 z 2a + 2b + 2c + 2x + 2y + 2z , so we subtract 4 y + 4 b 4y + 4b from 2 a + 6 b + 2 c + 2 x + 6 y + 2 z 2a + 6b + 2c + 2x +6y + 2z :

2 a + 2 b + 2 c + 2 x + 2 y + 2 z = ( 2 a + 6 b + 2 c + 2 x + 6 y + 2 z ) ( 4 y + 4 b ) 2a + 2b + 2c + 2x + 2y + 2z = (2a + 6b + 2c + 2x +6y + 2z) - (4y + 4b)

2 a + 2 b + 2 c + 2 x + 2 y + 2 z = 36 8 2a + 2b + 2c + 2x + 2y + 2z = 36 - 8

2 a + 2 b + 2 c + 2 x + 2 y + 2 z = 28 2a + 2b + 2c + 2x + 2y + 2z = \boxed{28}

Heather Thompson
Nov 7, 2017

I start with the inner rectangle. I can see it has, as one solution, equal sides of 1 unit each. I can then determine the remaining lengths:

(6-1-1)/2=2

(8-1-1)/2=3

(12-1-1)/2=5

I can now calculate the larger perimeter:

2((5+1+2) + (2+1+3))=28

This result can be checked by assigning different values to the original rectangle.

I'm glad I'm not the only one who figured it out this way

Jessie Paine - 3 years, 7 months ago
Mateusz Górak
Nov 12, 2017

Lol, I didn't think of rearranging the walls and I just proved it mathematicly XD x+y= 1 3 \frac{1}{3} (x+a)= 2 3 \frac{2}{3} (x+b) Therefore: E = 3(x+y)+x+y+ 3 2 \frac{3}{2} (x+y)= ... = 15 16 \frac{15}{16} x+ 11 2 \frac{11}{2} b AND x+y= 2 3 \frac{2}{3} (y+c)= 1 2 \frac{1}{2} (y+d) Therefore: F = 3 2 \frac{3}{2} (x+y)+x+y+2(x+y)= ... = 9 2 \frac{9}{2} + 3 2 \frac{3}{2} b Perimeter = 2E+2F=5x+11y+9x+3y=14x+14y If 4=2x+2y then 2=x+y and y=2-x Perimeter=14x+14(2-x)=14x-14x+28= 28

Nilesh Kulkarni
Nov 10, 2017

Perimeter of Rectangle = 2(l+b)

Kyle Smith
Nov 10, 2017

It could be several different answers. The perimeters are given but it doesn't say the sides must be whole. What if the middle rectangle is .5x1.5? The answer could be 30. If the drawing is in perfect scale, there is no way all of the sides are whole numbers.

True. But even with decimals. The length would be 9.5 and the width 4.5.

P=2(9.5+4.5) P still equals 28

Cece Dee - 3 years, 7 months ago
Brady McLean
Nov 9, 2017

The heights for all the middle "rectangles" are 0. Those are lines. The scaling for this diagram is atrocious.

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