推荐回答（1个）
由0＜∠B＜90°，∴0＜cosB＜1，
即0＜3m+1＜1，
-1＜3m＜0，
-1/3＜m＜0.
由tana=1/2＞0，y/x=1/2，∴y=x/2，
即k=1/2.（k就是y的斜率）。
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