設a,b,c,d為實數,下列何者為真? (1)a<b則a-b<0,a-b<0則a<b (2)b/a>0則ab>0,ab>0則b/a>0 (3)b/a>1則b>a,b>a則b/a>1 (4)若ac>0,b/a>d/c則bc>ad,bc>ad則b/a>d/c (5)若ac<0,b/a>d/c則bc<ad,bc<ad則b/a>d/c
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