证明:
过M作OP的垂线交OP于N
∠b = ∠a
MN = r * sin a
ON = r * cos a
PN = R - ON = R - r * cos a
∠c = arctan( MN / PN ) = arctan[ ( r * sin a) / (R - r * cos a) ]
∠d = (180 - ∠b - ∠c ) - 90
=90 - a - arctan[ (r * sin a) / ( R - r * cos a) ]
由于定义 ∠a在地平以下为负,用 -a 代入 上式 a,得:
d = 90 + a + arctan[ (r * sin a) / ( R - r * cos a) ]
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