如果三条直线mx+y+3=0,x﹣y﹣2=0,2x﹣y+2=0不能成为一个三角形三边所在的直线,求m的值.
解:①mx+y+3=0与x﹣y﹣2=0平行时,m=﹣1,此时满足题意,所以m=﹣1;
②mx+y+3=0与2x﹣y+2=0平行时,m=﹣2,此时满足题意,所以m=﹣2;
③联立x﹣y﹣2=0,2x﹣y+2=0得 x-y
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