※ 引述《ec1234 (淺草)》之銘言：
: http://i.imgur.com/gOMH3Um.jpg
: 謝謝
2.
P對AC做垂足E
=> PE // BA
PE = 9/2
延長PE及BQ使交於D
=> △QPD = △QAB
=> QD = QB
=> △QAD = △QPB
=> AD = BP = AP
=> AR = (2/3)AE
= (2/3)√[7.5^2 - 4.5^2]
= 4
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