※ 引述《jimmychad (吉米)》之銘言：
: 0<=a,b,c,d<=1
: 如何證明
: c<a*d
: 與
: b*c>d
: 相互矛盾
Suppose not, there are a, b, c & d with 0 ≦ a,b,c,d ≦ 1,
c < ad and bc > d. Then 0 ≦ c < ad, we have a > 0, and so
c < ad < abc, and so 1 < ab, there is at least one of a and b
strictly greater than 1.
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