∵m：y=x/2,∴直线m的斜率k=1/2设直线l：y=k'x+b∵直线m和l夹角为45°∴tan45°=|(k'-k)/(1+k'k)|即：±1=(k'-1/2)/(1+k'/2)得出：k'=3或k'=-1/3∵直线l是由m逆时针45°得到的∴k'＞k,∴k'=3又∵直线l过点P(2,1)带入...