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解法一:若直線斜率不存在,則方程為x=3.由{■(x=3"," @2x"-" y"-" 2=0"," )┤得A(3,4)., 由{■(x=3"," @x+y+3=0"," )┤得B(3,-6).由于(4+"(-" 6")" )/2=-1≠0,∴P不為線段AB的中點(diǎn).若直線斜率存在,設(shè)為k,則方程為y=k(x-3).由{■(y=k"(" x"-" 3")," @2x"-" y"-" 2=0"," )┤得A((3k"-" 2)/(k"-" 2),4k/(k"-" 2)).由{■(y=k"(" x"-" 3")," @x+y+3=0"," )┤得B((3k"-" 3)/(k+1),-6k/(k+1)).∵P(3,0)為線段AB的中點(diǎn),∴{■((3k"-" 2)/(k"-" 2)+(3k"-" 3)/(k+1)=6"," @4k/(k"-" 2) "-" 6k/(k+1)=0"." )┤∴{■(2k"-" 16=0"," @k^2 "-" 8k=0"." )┤∴k=8.∴所求直線方程為y=8(x-3),即8x-y-24=0.分析二:設(shè)出A(x1,y1),由P(3,0)為AB的中點(diǎn),易求出B的坐標(biāo),而點(diǎn)B在另一直線上,從而求出x1、y1的值,再由兩點(diǎn)式求直線的方程.解法二:設(shè)A點(diǎn)坐標(biāo)為(x1,y1),則由P(3,0)為線段AB的中點(diǎn),得B點(diǎn)坐標(biāo)為(6-x1,-y1).∵點(diǎn)A,B分別在已知兩直線上,∴{■(2x_1 "-" y_1 "-" 2=0"," @"(" 6"-" x_1 ")" +"(-" y_1 ")" +3=0"." )┤解得{■(x_1=11/3 "," @y_1=16/3 "." )┤∴A 11/3,16/3 .∵點(diǎn)A,P都在直線AB上,∴直線AB的方程為(y"-" 0)/(16/3 "-" 0)=(x"-" 3)/(11/3 "-" 3),即8x-y-24=0.分析三:由于P(3,0)為線段AB的中點(diǎn),可對(duì)稱地將A,B坐標(biāo)設(shè)為(3+a,b),(3-a,-b),代入已知方程.
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