please help me to deal this question~~ thanks~~five touching circles each have radius 1 and their centres are at the vertices of a regular pentagon . What is the radius of the circle through the touching points? A.tan18 B.tan36 C. tan45 D.tan54

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please help me to deal this question~~ thanks~~
five touching circles each have radius 1 and their centres are at the vertices of a regular pentagon . What is the radius of the circle through the touching points?
A.tan18 B.tan36 C. tan45 D.tan54 E tan 72

应该是 B

tan54
each angle of the pentagon is 108 degrees
then an right angle edge of a right triangle is 1 in length ,and this edge is a circles radius
then tan54=l/1 l=tan54

说汉语，

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