If the length of AB is 20cm.

What is the area between the two circles?

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- May 8th 2006, 01:13 PMNatasha1What is the area between the two circles?
If the length of AB is 20cm.

What is the area between the two circles? - May 8th 2006, 07:01 PMCaptainBlackQuote:

Originally Posted by**Natasha1**

the radius of the smaller circle $\displaystyle r_1$ and the radius of the outer circle $\displaystyle r_2$.

Then triangle $\displaystyle OMB$ is a right triangle with hypotenuse $\displaystyle r_2$

and the other sides $\displaystyle r_1$, and $\displaystyle 10$. Therefore by Pythagoras:

$\displaystyle

r_2^2=10^2+r_1^2

$,

or:

$\displaystyle

r_2^2-r_1^2=100

$

The area of the annulus is:

$\displaystyle

\pi r_2^2 - \pi r_1^2=\pi (r_2^2-r_1^2)=\pi.100

$

RonL