※ 引述《pml0415 (拼命)》之銘言：
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4. |2ax-b|<2x^2-8x+9 for all x.
let x=2, then |4a-b|< 8-16+9=1.
as a,b, are natural numbers, we have |4a-b|=0, ie. b=4a.
let x=1, |2a-b|<3, |-2a|<3, |a|<1.5 => a=1.
Thus, b=4a=4.
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