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<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">解：∵<i>BE</i>⊥<i>AD</i>于<i>E</i>，<i>CF</i>⊥<i>AD</i>于<i>F</i>，</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">∴<i>CF</i>∥<i>BE</i>，</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">∴∠<i>DCF</i>＝∠<i>DBE</i>，设∠<i>DCF</i>＝∠<i>DBE</i>＝α，</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">∴<i>CF</i>＝<i>DC</i>•cosα，<i>BE</i>＝<i>DB</i>•cosα，</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">∴<i>BE</i>+<i>CF</i>＝（<i>DB</i>+<i>DC</i>）cosα＝<i>BC</i>•cosα，</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">∵∠<i>ABC</i>＝90°，</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">∴<i>O</i>＜α＜90°，</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">当点<i>D</i>从<i>B</i>向<i>C</i>运动时，α是逐渐增大的，</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">∴cosα的值是逐渐减小的，</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">∴<i>BE</i>+<i>CF</i>＝<i>BC</i>•cosα的值是逐渐减小的．</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">故选<i>C</i>．</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">面积法：<i>S</i></span></span><sub><span style="font-size:12.0pt"><span style="line-height:150%"><span style="font-family:宋体">△<i>ABC</i></span></span></span></sub><span style="line-height:150%"><span style="font-family:宋体">＝</span></span></span></span></span><img height="35" src="/uploads/meth/20210402/553527829ebd5e4e8ce4de1c57ebb553.png" width="13" /><span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">•<i>AD</i>•<i>CF</i>+</span></span></span></span></span><img height="35" src="/uploads/meth/20210402/e51256632b005a08f0fde12d07b20c2d.png" width="13" /><span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">•<i>AD</i>•<i>BE</i>＝</span></span></span></span></span><img height="35" src="/uploads/meth/20210402/66d37caa0b54ee982a069f3e73fe1302.png" width="13" /><span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">•<i>AD</i>（<i>CF</i>+<i>BE</i>），</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">∴<i>CF</i>+<i>BE</i>＝</span></span></span></span></span><img height="41" src="/uploads/meth/20210402/adb9ea94865954a3215c516a522e55d6.png" width="55" /><span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">，</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">∵点<i>D</i>沿<i>BC</i>自<i>B</i>向<i>C</i>运动时，<i>AD</i>是增加的，</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">∴<i>CF</i>+<i>BE</i>的值是逐渐减小．</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:宋体">故选：<i>C</i>．</span></span></span></span></span><br />
<img height="118" src="/uploads/meth/20210402/06813f12a9f75e10a9bb99d7e46b619a.png" width="132" /><br />

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