Is It A Square?

Geometry Level 1

In the above semicircle, we mark off lengths of 1, 3 and 1 along the diameter. We then draw a rectangle on side length 3, till it touches the semicircle

What is the area of the green rectangle?

6 7 8 9

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6 solutions

Michael Huang
Dec 19, 2016

Here is the following illustration as the start:

Because of line length symmetry for A E \overline{AE} , we can conclude that A C = 2.5 |AC| = 2.5 , which is the radius of the semicircle. Thereby, F C = G C = 2.5 |FC| = |GC| = 2.5 .

Because of midpoint C C , we see that B C = A C A B = 1.5 |BC| = |AC| - |AB| = 1.5 and also C D = C E D E = 1.5 |CD| = |CE| - |DE| = 1.5 . Since Δ B F C \Delta BFC is the right triangle (as well as Δ C G D \Delta CGD ), Pythagorean theorem shows that F B 2 + B C 2 = F C 2 F B = F C 2 B C 2 = 2 \begin{array}{rl} |FB|^2 + |BC|^2 &= |FC|^2\\ |FB| &= \sqrt{|FC|^2 - |BC|^2}\\ &= 2 \end{array} which thus shows that the area of the rectangle is A = F B × B D = 6 A = |FB| \times |BD| = \boxed{6} .

Good explanation. Also your diagram is very clear. What tool did you use to draw it?

Richard Costen - 4 years, 5 months ago

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Geogebra . :)

Michael Huang - 4 years, 5 months ago

I think the problem has bad multiple choices. without trying to solve the problem first, I know that the triangle area is 6 (without knowing why). Maybe, you could change them to multiplication of 3.

Shufi Khanza - 4 years, 5 months ago

In the top right of the diagram, it should be |BC|=1.5 or |BD|=3 ...... NOT |BC|=3

Bob Kadylo - 4 years, 5 months ago

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I have fixed the diagram! Thanks for catching the typo! :)

Michael Huang - 4 years, 5 months ago

cool I couldn't figure this one out

Charlotte Milanese - 4 years, 5 months ago

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..and now you know how to work out this problem! :)

Michael Huang - 4 years, 5 months ago

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no ! lol ! I still can't figure it out

Charlotte Milanese - 4 years, 5 months ago
汶良 林
Jan 31, 2017

Oh, very nice flipping!

Chung Kevin - 4 years, 4 months ago

x = 2. 5 2 1. 5 2 = 2 x=\sqrt{2.5^2-1.5^2}=2 u n i t s units

a r e a = 2 ( 3 ) = 6 area = 2(3) = 6 s q u a r e square u n i t s units

Vijay Simha
Jan 20, 2017

The vertical side of the rectangle is essentially the geometric mean of two numbers: 1 and 3 +1 = 4 ie. the square root of 1 x 4 = 2.

It follows that the areas of the rectangle is therefore 2 x 3 = 6

Oh, that's a quick trick to find the vertical side of the rectangle using power of a point . Very nice :)

Chung Kevin - 4 years, 4 months ago

Can you explain why the vertical line should be the geometric mean of these two lengths? I also cannot see how this relates to the power of a point...

Frederick Wells - 4 years, 4 months ago

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Look at this http://www.cut-the-knot.org/pythagoras/GeometricMean.shtml

and this: https://en.wikipedia.org/wiki/Semicircle

You can derive this yourself with elementary applications of Pythagoras theorem.

Vijay Simha - 4 years, 4 months ago

It's only known as the 2 secant theorem, which says that if AB and CD are chords of a circle that intersect at P, then P A × P B = P C × P D PA \times PB = PC \times PD .

So, in this problem, we take A B AB to be the diameter of the semicircle, and C D CD to be the vertical line (but remember that we need a circle and not a semicircle). So, point P because the left corner of the rectangle, and we get P A × P B = 1 × 4 PA \times PB = 1 \times 4 .

Chung Kevin - 4 years, 4 months ago
Arjun Gupta
Feb 8, 2017

diameter is 5. Hence radius is 2.5

Now from centre to one edge of the rectangle forms a right triangle with H=2.5 with B=1.5. So P=2. Area is 3*2

Pythagoras theorem to the rescue!

Chung Kevin - 4 years, 4 months ago
Erik Sikora
Jan 31, 2017

Add 1+3+1=5 the diameter of the circle. Thus 2.5 is the radius. Half of 3 is 1.5. Now a right triangle measuring 1.5 on one leg and 2.5 on the Hypotenuse is formed. Noticing a pattern, multiply by 10 and divide by 5 reveals a 3-4-5 right triangle. Thus 4×5/10=2. 2×3=6 the area of the rectangle.

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