If the length of AB is 20cm.
What is the area between the two circles?
Label the centre of the circle $\displaystyle O$, the midpoint of $\displaystyle AB$ $\displaystyle M$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