<p>There's a graph to this question. Can also help me out? I don't get why its 18pi, shouldn't it be 36pi?</p>
<p>C=36pi, and C=2rpi so therefore r=18. It's asking for the length of the curved path from A to B right? The curved path from A to B equals AO+OB, so it should be r+r=36pi. Why is it 18pi?</p>
<p>Thanks in advance. (You'll need to see the graph on the book to get what i'm saying)</p>
<p>the radius of each smaller circle is 9, so the total circumference of each smaller on is 2(9)pi = 18pi. However each is a semi circle or 9pi each, so 9pi + 9pi = 18pi</p>
<p>Just another tricky solution: Moving the lower (OB) semi-circle upward to fit the upper (OA) semi-circle, you'll see that these two semi-cirles compose a circle with diametre OA=1/2 AB = the length of the curved path we're considering.
Since: circumference of larger circle (AB) = 2 Pi R
circumference of smaller circle (OA) = 2 Pi . 1/2 R = Pi R</p>
<p>====> curve A-B = 1/2 of 36 Pi = 18 Pi</p>
<p>My reasoning appears a lil verbose, but if you attack the problem this way, you'll see how quick this solution is. In fact, it takes me fewer than 5 seconds to pick C as the answer!</p>
<p>btw, the solution nd09 offers is great ^^</p>
<p>Thanks!!! XD</p>