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Two squares, each of side lengths 1 unit and having their [#permalink]

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14 Sep 2010, 07:27

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Two squares, each of side lengths 1 unit and having their centres at O, are rotated with respect to each other to generate octagon ABCDEFGH, as shown in the figure above. If AB = 43/99, find the area of the octagon.

Two squares, each of side lengths 1 unit and having their centres at O, are rotated with respect to each other to generate octagon ABCDEFGH, as shown in the figure above. If AB = 43/99, find the area of the octagon.

Attachment:

G86.PNG

a. 73/99 b. 86/99 c. 1287/9801 d. 76/99

First note that all the triangles will have the same dimensions. Let the legs of these triangles be \(x\) and \(y\), also we know that the hypotenuse equals to \(\frac{43}{99}\), so \(x^2+y^2=(\frac{43}{99})^2\).

If we take one side of a square we'll see that \(x+y+\frac{43}{99}=1\) --> \(x+y=\frac{56}{99}\) --> square it --> \(x^2+2xy+y^2=(\frac{56}{99})^2\), as from above \(x^2+y^2=(\frac{43}{99})^2\), then: \((\frac{43}{99})^2+2xy=(\frac{56}{99})^2\) --> \(2xy=(\frac{56}{99})^2-(\frac{43}{99})^2=(\frac{56}{99}-\frac{43}{99})(\frac{56}{99}+\frac{43}{99})=\frac{13}{99}*1=\frac{13}{99}\);

Now, the are of the octagon will be area of a square minus area of 4 triangles --> \(area=1-4*(\frac{1}{2}*xy)=1-2xy=1-\frac{13}{99}=\frac{86}{99}\).

EASY WAY is as shown below AREA = .5*base*hiegt = 0.5*43/99 * 1/2 note: i/2 is the hieght as the length of a side of the given square is 1 and half of that is what is the hieght

This doesnt looks like a straight line i dont think i can be 0.5

EASY WAY is as shown below AREA = .5*base*hiegt = 0.5*43/99 * 1/2 note: i/2 is the hieght as the length of a side of the given square is 1 and half of that is what is the hieght

This doesnt looks like a straight line i dont think i can be 0.5

Attachment:

G86.PNG

It is the length of the perpendicular from the center of the square to its side AB. It has to be half the length of the side, hence 1/2.
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Re: Two squares, each of side lengths 1 unit and having their [#permalink]

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12 Dec 2013, 12:28

Basically, we have to find the area of the square and the eight triangles that border it.

We are told that measure AB = 43/99. Therefore, we know that measure XA +BZ (see attached diagram) = 56/99. We also know that each triangle formed is a right triangle so we can use the Pythagorean theorem: a^2 + b^2 = c^2 or in this case, a^2 + b^2 = (43/99)^2. Also, keep in mind that the lengths XA + BA are also the leg lengths of the two right triangles.

What we know:

a+b = 56/99 a^2 + b^2 = (43/99)^2

From the first equation, we could isolate a variable to get a=(56/99) - b then plug it into the Pythagorean theorem but that might get messy. Instead, we notice that in both equations we have an a + b: we can square a and b in the first equation to make it easier to substitute into the second equation.

a+b = 56/99 (a+b)^2 = (56/99)^2 a^2 + 2ab + b^2 = (56/99)^2 a^2 + b^2 + 2ab = (56/99)^2 Now we have an equation that can easily plug into the Pythagorean theorem

Clearly I made a mistake here. If I were to square both sides I would get 169/9801 which is much smaller than 13/99. If I were to square (56/99)^2 and (43/99)^2 then reduce, I would get 13/99 but this is far too time consuming for the test. Where did I go wrong with my equation?

Furthermore, why would the area of this square be the square MINUS the area of the triangles? Wouldn't it be the area of the square PLUS the area of the triangles?

Thanks!

Attachments

EXAMPLE SEVEN.png [ 18.91 KiB | Viewed 2677 times ]

Two squares, each of side lengths 1 unit and having their centres at O, are rotated with respect to each other to generate octagon ABCDEFGH, as shown in the figure above. If AB = 43/99, find the area of the octagon.

Attachment:

G86.PNG

a. 73/99 b. 86/99 c. 1287/9801 d. 76/99

First note that all the triangles will have the same dimensions. Let the legs of these triangles be \(x\) and \(y\), also we know that the hypotenuse equals to \(\frac{43}{99}\), so \(x^2+y^2=(\frac{43}{99})^2\).

If we take one side of a square we'll see that \(x+y+\frac{43}{99}=1\) --> \(x+y=\frac{56}{99}\) --> square it --> \(x^2+2xy+y^2=(\frac{56}{99})^2\), as from above \(x^2+y^2=(\frac{43}{99})^2\), then: \((\frac{43}{99})^2+2xy=(\frac{56}{99})^2\) --> \(2xy=(\frac{56}{99})^2-(\frac{43}{99})^2=(\frac{56}{99}-\frac{43}{99})(\frac{56}{99}+\frac{43}{99})=\frac{13}{99}*1=\frac{13}{99}\);

Now, the are of the octagon will be area of a square minus area of 4 triangles --> \(area=1-4*(\frac{1}{2}*xy)=1-2xy=1-\frac{13}{99}=\frac{86}{99}\).

Answer: B.

hi Banuel. I can't understand why I should subtract the four triangles when it seems that I should add them to the area of the square. Please explain. Thanks.

Two squares, each of side lengths 1 unit and having their centres at O, are rotated with respect to each other to generate octagon ABCDEFGH, as shown in the figure above. If AB = 43/99, find the area of the octagon.

Attachment:

The attachment G86.PNG is no longer available

a. 73/99 b. 86/99 c. 1287/9801 d. 76/99

First note that all the triangles will have the same dimensions. Let the legs of these triangles be \(x\) and \(y\), also we know that the hypotenuse equals to \(\frac{43}{99}\), so \(x^2+y^2=(\frac{43}{99})^2\).

If we take one side of a square we'll see that \(x+y+\frac{43}{99}=1\) --> \(x+y=\frac{56}{99}\) --> square it --> \(x^2+2xy+y^2=(\frac{56}{99})^2\), as from above \(x^2+y^2=(\frac{43}{99})^2\), then: \((\frac{43}{99})^2+2xy=(\frac{56}{99})^2\) --> \(2xy=(\frac{56}{99})^2-(\frac{43}{99})^2=(\frac{56}{99}-\frac{43}{99})(\frac{56}{99}+\frac{43}{99})=\frac{13}{99}*1=\frac{13}{99}\);

Now, the are of the octagon will be area of a square minus area of 4 triangles --> \(area=1-4*(\frac{1}{2}*xy)=1-2xy=1-\frac{13}{99}=\frac{86}{99}\).

Answer: B.

hi Banuel. I can't understand why I should subtract the four triangles when it seems that I should add them to the area of the square. Please explain. Thanks.

Look at the figure below...the area of octagon is the coloured portion which is 1 - area of 4 similar triangles

The drawing is not to scale but I hope you get the point

Attachments

untitled.PNG [ 4.17 KiB | Viewed 2629 times ]

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Re: Two squares, each of side lengths 1 unit and having their [#permalink]

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06 Jul 2014, 09:29

Bunuel wrote:

RaviChandra wrote:

Two squares, each of side lengths 1 unit and having their centres at O, are rotated with respect to each other to generate octagon ABCDEFGH, as shown in the figure above. If AB = 43/99, find the area of the octagon.

Attachment:

G86.PNG

a. 73/99 b. 86/99 c. 1287/9801 d. 76/99

First note that all the triangles will have the same dimensions. Let the legs of these triangles be \(x\) and \(y\), also we know that the hypotenuse equals to \(\frac{43}{99}\), so \(x^2+y^2=(\frac{43}{99})^2\).

If we take one side of a square we'll see that \(x+y+\frac{43}{99}=1\) --> \(x+y=\frac{56}{99}\) --> square it --> \(x^2+2xy+y^2=(\frac{56}{99})^2\), as from above \(x^2+y^2=(\frac{43}{99})^2\), then: \((\frac{43}{99})^2+2xy=(\frac{56}{99})^2\) --> \(2xy=(\frac{56}{99})^2-(\frac{43}{99})^2=(\frac{56}{99}-\frac{43}{99})(\frac{56}{99}+\frac{43}{99})=\frac{13}{99}*1=\frac{13}{99}\);

Now, the are of the octagon will be area of a square minus area of 4 triangles --> \(area=1-4*(\frac{1}{2}*xy)=1-2xy=1-\frac{13}{99}=\frac{86}{99}\).

Answer: B.

Hi Bunuel,

I don't understand why we subtract only 4 triangle? Aren't we supposed to subtract 8 triangles?

Two squares, each of side lengths 1 unit and having their centres at O, are rotated with respect to each other to generate octagon ABCDEFGH, as shown in the figure above. If AB = 43/99, find the area of the octagon.

Attachment:

The attachment G86.PNG is no longer available

a. 73/99 b. 86/99 c. 1287/9801 d. 76/99

First note that all the triangles will have the same dimensions. Let the legs of these triangles be \(x\) and \(y\), also we know that the hypotenuse equals to \(\frac{43}{99}\), so \(x^2+y^2=(\frac{43}{99})^2\).

If we take one side of a square we'll see that \(x+y+\frac{43}{99}=1\) --> \(x+y=\frac{56}{99}\) --> square it --> \(x^2+2xy+y^2=(\frac{56}{99})^2\), as from above \(x^2+y^2=(\frac{43}{99})^2\), then: \((\frac{43}{99})^2+2xy=(\frac{56}{99})^2\) --> \(2xy=(\frac{56}{99})^2-(\frac{43}{99})^2=(\frac{56}{99}-\frac{43}{99})(\frac{56}{99}+\frac{43}{99})=\frac{13}{99}*1=\frac{13}{99}\);

Now, the are of the octagon will be area of a square minus area of 4 triangles --> \(area=1-4*(\frac{1}{2}*xy)=1-2xy=1-\frac{13}{99}=\frac{86}{99}\).

Answer: B.

Hi Bunuel,

I don't understand why we subtract only 4 triangle? Aren't we supposed to subtract 8 triangles?

Attachment:

Untitled.png [ 7.68 KiB | Viewed 2040 times ]

The area of yellow octagon = the area of blue square - the area of 4 red triangles.

Re: Two squares, each of side lengths 1 unit and having their [#permalink]

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23 Sep 2014, 08:04

RaviChandra wrote:

muralimba wrote:

EASY WAY is as shown below AREA = .5*base*hiegt = 0.5*43/99 * 1/2 note: i/2 is the hieght as the length of a side of the given square is 1 and half of that is what is the hieght

This doesnt looks like a straight line i dont think i can be 0.5

Attachment:

G86.PNG

Can anyone input if this method is correct? I am also not sure that the line has to be straight. If the length of the hypotenuse of the triangle was more that half of the side of the square than I could agree. but it's not...

Re: Two squares, each of side lengths 1 unit and having their [#permalink]

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