find the area of the ring between two circles

Question

133 · Answer

Where are the circles.

133 · Answer

Find the diameter first. how much is that?

nincompoop · Answer

|dw:1383950470210:dw|

nincompoop · Answer

there's no measurement given

isaiah.feynman · Answer

The area between two circles or an annulus is given by the formula $$A = \pi(R^{2}-r^{2})$$Where R is radius of the bigger circle and r is radius of the smaller circle.

133 · Answer

|dw:1383950857236:dw| Just measure it where the line is on the small circle how long is that? Then how long is the big circle? If that doesn't help then I can't do any more sorry.

anonymous · Answer

|dw:1383951106113:dw|
Pythagoras says $r^2 + (\frac{1}{2}a)^2 = R^2$
Multiply both sides by $\pi$

$$\pi r^2 + \frac{\pi}{4}a^2 = \pi R^2$$$$\frac{\pi}{4}a^2 = \pi(R^2 - r^2)$$
Which, as shown above, is the area.

nincompoop · Answer

I was trying to make someone use calculus and see the struggle... LOL