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hello everyone i am looking at following problem from this link

belt image

it is possible to say that everything is explained detailed but i did not understand some of them 1.what about number 7?how is here written 7 without explanation what does it mean?then why is here mentioned semicircles i am confused about it it is drawn simple two circles please help me to understand everything given in above mentioned task

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[Meta: This is at least the second question you ask from that page. It looks like a pretty unreliable source.] I don't understand why they are talking about semi-circles. However, the number $7 = 13.5-6.5$ is motivated by applying Pythagoras to determine the length of the blue line. (we know the lengths of the hypotenuse and the short leg) – t.b. Jul 7 '11 at 9:15
up vote 4 down vote accepted

The reason you don't understand some of it is that it's wrong :-) The length of the belt is not given by two semicircles and two tangents; the belt follows the larger circle for more than a semicircle and the smaller circle for less than a semicircle, and the sum of these two contributions is not the sum of the lengths of two semicircles. For the correct solution you need to add $$2(13.5-6.5)\alpha=2(13.5-6.5)\arccos(24/25)\approx4$$ to the given incorrect solution, where $\alpha$ is the angle by which the belt follows the larger circle beyond a semicircle on each side.

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thanks very much – dato datuashvili Jul 7 '11 at 9:22

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