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#1 2007-03-07 16:16:00

Dharshi
Member
Registered: 2006-10-31
Posts: 56

What is the answer?

The bull's eye of a target is a circle 20 cm
in diameter. It is surrounded by four rings each 15 cm wide. Find the area of each ring.

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#2 2007-03-07 16:22:24

Jai Ganesh
Administrator
Registered: 2005-06-28
Posts: 45,954

Re: What is the answer?

The rings so formed are called Annulus.
The method to calculate the area of each annulus would be as follows:-
The first cirle after the bull's eye has a radius of 17.5 cm.
The bull's eye has a radius of 10 cm.
Hence, the area of the first ring would be

.
Extrapolate and you get the area of the other rings.


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#3 2007-03-07 16:23:18

JaneFairfax
Member
Registered: 2007-02-23
Posts: 6,868

Re: What is the answer?

If two concentric circles have radii r[sub]1[/sub] and r[sub]2[/sub] (r[sub]1[/sub] < r[sub]2[/sub]), the area of the ring in between (called an annulus) is π(r[sub]2[/sub][sup]2[/sup]−r[sub]1[/sub][sup]2[/sup]).
­
And ganesh, the 17.5 is wrong – it should be 12.5.

Last edited by JaneFairfax (2007-03-11 05:44:37)

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#4 2007-03-07 16:27:34

Stanley_Marsh
Member
Registered: 2006-12-13
Posts: 345

Re: What is the answer?

Think harder! the solution will be obvious when you draw the graph


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#5 2007-03-07 20:24:14

lightning
Real Member
Registered: 2007-02-26
Posts: 2,060

Re: What is the answer?

how do you do that


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#6 2007-03-07 22:35:45

mathsyperson
Moderator
Registered: 2005-06-22
Posts: 4,900

Re: What is the answer?

Incidentally, you can also work out the area of an annulus by finding the length of the red line shown below, then finding the area of a circle with that length as the diameter.

Of course, finding that length in the first place is quite tricky, so that method might not be too useful.

annulusns7.png


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It wanted to be normal.

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