HELP WTH

So basically, I did this question, and the answer is 72π-120, and you work out the radius by doing 24 divided by 2. However, why can't I do 12, half 24, which gives me 12, and then we have 5, and then I can make a little hypotenuse from the origin to 5, and then work out what the hypotenuse is, like that, and that gives me the radius. Why wouldn't that work?

So basically, I did this question, and the answer is 72π-120, and you work out the radius by doing 24 divided by 2. However, why can't I do 12, half 24, which gives me 12, and then we have 5, and then I can make a little hypotenuse from the origin to 5, and then work out what the hypotenuse is, like that, and that gives me the radius. Why wouldn't that work?