PM?PO2?OM2?42?32?5，PN?PO2?ON2?42?42?42．

11V?Sh???8?6??4?64．

33（2）S侧?2S△PAB?2S△PBC?AB?PM?BC?PN?8?5?6?42?40?242．

321．【答案】h2?19h． 3【解析】当锥顶向上时，设圆锥底面半径为r，水的体积为：11?2?219V??r2h???r??h??r2h．

33?3?38121当锥顶向下时，设水面圆半径为r′，则V???r'2?h2．

3?h23r21h22r2h2r又r'?，此时V???2?h2?，

3hh3h2∴

?h23r23h2331921919??rh，∴h2?h． h，即所求h2的值为813322．【答案】（1）AD?36cm；（2）V?50435?cm3

??

【解析】（1）设圆台上、下底面半径分别为r、R，AD＝x，则OD?72?x， 60????72?R?12?2?R?由题意得?，∴?．即AD应取36cm． 180x?36???72?x?3R（2）∵2?r????OD??36，∴r?6cm， 3322圆台的高h?x2??R?r??362??12?6??635．

11∴V??hR2?Rr?r2???635?122?12?6?62?50435?cm3

33??????