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180 ( n 2)

123AnA4

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R

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r

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.

R

r

rR

a

rR

a

Rra

rRa1. a=3

2. R = 3

3. r = 3

rRa1.a = 4

2. R = 4

3. r = 4

Rra1. a = 1

2. R = 2

3. r = 3