Not a perfect circle!

Geometry Level 2

Find the area of the blue shaded region in this 14 × 7 14\times 7 rectangle, with two semicircles of radius 7 drawn.


The answer is 60.19.

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6 solutions

Mas Mus
Apr 7, 2016

Look at the picture!

Δ A B C \Delta~ABC and Δ A C D \Delta~ACD are two congruent equilateral with side lenght r = 7 r=7 . So, the area of Δ A B C = Δ A C D = 49 3 4 \Delta ~ABC = \Delta ~ ACD = \frac{49\sqrt{3}}{4} . The area of sector A B C = 49 π 6 ABC = \frac{49\pi}{6} .

Then, the area of blue shaded region is

2 × 49 3 4 + 4 ( 49 π 6 49 3 4 ) = 98 π 3 49 3 2 = 60.19 \large{2\times{\frac{49\sqrt{3}}{4}}+4\left(\frac{49\pi}{6}-\frac{49\sqrt{3}}{4}\right)=\frac{98\pi}{3}-\frac{49\sqrt{3}}{2}=60.19}

How did u get 49pi/6? This part is confusing me

Jeff Lin - 2 years, 7 months ago
Robert DeLisle
Jul 19, 2017

Edwin Gray
Jan 16, 2018

Define a coordinate system with (0,0) at the mid-point of the bottom. Then the mid-point of the top is (0,7). The equations of the circles are: (1) x^2 + y^2 = 49, and (2) x^2 + (y - 7)^2 =49, or x^2 + y^2 - 14y + 49 = 49.Cancelling and substituting, we find the circles intersect at y = 7/2, and x = +/- sqrt(147)/2 = +/- 6.062178.. Then the area can be written as 4 integral from x=0 to x = 6.062178 of sqrt(49 - x^2)dx - 4 integral from x =0 to x = 6.062178 of 3.5dx . Carrying out the integration, Area = 60.19. Ed Gray

Ruiling Ge
Apr 27, 2015

Draw a line between the intersection points of the two circles, and draw a new half circle with the line on a separate paper. (just one of the half circles) the top part of our newly drawn graph is half of the area of the blue shaded area. We can solve by finding the center of the circle and connect it to the line to make a triangle. The height of the triangle will be 7 / 2 = 3.5 7/2=3.5 , hypotenuse will be 7 7 . This is one of our familiar 30 : 60 : 90 30:60:90 triangles, the base of our big triangle will be 7 3 × 2 7 \sqrt{3} \times 2 . The obtuse angle of the big triangle will be 180 30 30 = 120 180 -30 -30=120 . The area of our top half circle could be found by subtracting the area of the triangle from the cone π 7 2 × 120 / 360 7 / 2 × 7 3 × 2 = 30.095 \pi 7^{2} \times 120 /360 - 7/2 \times 7 \sqrt{3} \times 2 =30.095 The shaded area will be that doubled, which is 60.19 \boxed{60.19}

integrating would give perfect answer

Sanjoy Roy - 6 years ago

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Please show me how, Sanjoy.

Bee La Salle - 4 years, 6 months ago

Why u have taken obtuse triangle rather u Can do it directly by just using formula of area of circle

Aman Verma - 5 years, 5 months ago

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Please show us how, Aman

Bee La Salle - 4 years, 6 months ago

Why u have taken the Base radius as 7root2/2

Aman Verma - 5 years, 5 months ago

'tis very nice. Very clever, Mas Mus. I struggled with it. Got it wrong three times. Thanks.

Bee La Salle - 4 years, 6 months ago
Seyed Seyedy
Oct 1, 2019

Klaus Kübbeler
Apr 5, 2017

Solving by integration

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