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證明:假設X²;+2Y和Y²;+2X都小於-1 即:X²;+2Y<-1 Y²;+2X<-1 兩式相加得X²;+2Y+Y²;+2X<-2 X²;+2Y+Y²;+2X+1+1<0 (X+1)²;+(Y+1)²;<0 而(X+1)²;,(Y+1)²;均為非負數,其和應該不小於0,與上式衝突 所以假設不成立,原命題得證!
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