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Pat-piên-hìn
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Pat-piên-hìn (Yîn-ngî: Octagon) he chṳ́ yit-ke yù pat-thiàu siông-thùng chhòng-thu ke piên lâu pat-ke siông-thùng thai-séu ke kok kèu-sṳ̀n ke Chang Tô-piên-hìn. Chhai yit-ke chang pat piên hìn lî, mî yit-ke kok ke thai-séu tû-he 3π/4rad, chit tén-yî 135thu. Kì ke sṳ̂ lòi fû li fù-ho he {8}. Tui yî yit-ke piên-chhòng he a ke chang pat piên hìn, kì ke mien-chit yì hâ:
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