证明题(1949年清华大学

已知PA,PB,PC为过圆周上点P三弦,PT为圆之切线,设有一直线与PT平行,交PA,PB,PC于A',B',C'三点.

求证:PA∙PA'=PB∙PB'=PC∙PC'.

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讨论

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n 多边形诸角之和=______.