解ï¼è®¾BF=xï¼åFC=12-xï¼äºæ¯â³BDFçé¢ç§¯S₁=(1/2)Ã12x=6xï¼
â³FACçé¢ç§¯S₂=(1/2)Ã12(12-x)=72-6xï¼
æ é´å½±é¨åçé¢ç§¯=S₁+S₂-2(å边形EFGHçé¢ç§¯)=6x+(72-6x)-12=72-12=60cm².
解ï¼é´å½±é¨åçé¢ç§¯=(1/2)Ã12Ã4+(1/2)Ã12Ã4-2Ã4=12Ã4-2Ã4=40cm².
解ï¼è®¾â³EDFçé¢ç§¯ä¸ºS₁ï¼â³ABEçé¢ç§¯ä¸ºS₂ï¼DF=xï¼DE=yï¼
å·²ç¥S₁-S₂=75ï¼æ 2(S₁-S₂)=150............(1)
ç±äºDE/DF=BE/ABï¼å³æy/x=(15-y)/15ï¼
äºæ¯æ15y=(15-y)xï¼(x+15)y=15xï¼y=15x/(x+15)..........(2)ï¼
2S₁=xyï¼2S₂=15(15-y)ï¼ä»£å ¥(1)å¼å¾ï¼xy-15(15-y)=xy-225+15y=(x+15)y-225=150ï¼
(x+15)y=375ï¼æ å¾y=375/(x+15).........(3)
ç±(2)(3)å¾15x(x+15)=375/(x+15)ï¼æ å¾15x=375ï¼â´x=375/15=25cm.
å³DFçé¿æ¯25cm.
谢谢