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SEOUL, Sept. 26 (Xinhua) — Hundreds of families from the two Koreas on Saturday met at the DPRK's (朝鲜人民主主义人民共和国) Mount Kumgang resort (度假胜地)in the first government-arranged reunion since the inauguration（就职）of South Korean President Lee Myung-bak.     Earlier in the day, 96 South Koreans, of whom more than three-quarters were at the age of 70 or older, crossed the inter-Korean border（韩朝边界）, heading for the Kumgang resort to meet their families separated by the 1950—1953 Korean War.     Arriving at the DPRK's resort in the afterno如图，PA与圆O相切于A，PCB为圆O的割线，并且不过圆心O，已知∠BPA=30°，，PC=1，则圆O的半径等于    ．