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?
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Good explanation. Also your diagram is very clear. What tool did you use to draw it?
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.
In the top right of the diagram, it should be |BC|=1.5 or |BD|=3 ...... NOT |BC|=3
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I have fixed the diagram! Thanks for catching the typo! :)
cool I couldn't figure this one out
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..and now you know how to work out this problem! :)
Oh, very nice flipping!
x = 2 . 5 2 − 1 . 5 2 = 2 u n i t s
a r e a = 2 ( 3 ) = 6 s q u a r e u n i t s
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 :)
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...
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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.
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 .
So, in this problem, we take A B to be the diameter of the semicircle, and C D 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 .
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!
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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Here is the following illustration as the start:
Because of line length symmetry for A E , we can conclude that ∣ A C ∣ = 2 . 5 , which is the radius of the semicircle. Thereby, ∣ F C ∣ = ∣ G C ∣ = 2 . 5 .
Because of midpoint C , we see that ∣ B C ∣ = ∣ A C ∣ − ∣ A B ∣ = 1 . 5 and also ∣ C D ∣ = ∣ C E ∣ − ∣ D E ∣ = 1 . 5 . Since Δ B F C is the right triangle (as well as Δ C G D ), Pythagorean theorem shows that ∣ F B ∣ 2 + ∣ B C ∣ 2 ∣ F B ∣ = ∣ F C ∣ 2 = ∣ F C ∣ 2 − ∣ B C ∣ 2 = 2 which thus shows that the area of the rectangle is A = ∣ F B ∣ × ∣ B D ∣ = 6 .