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設t=X^2+2x+a,要使值域為全體實數,則t要能取到所以正實數,而t=(x+1)^2+a-1.且圖形對稱軸為x=-1,開口向上,與y軸交點為(0,a-1)所以當a-1≤0即a≤1時存在x的取值範圍使t能取到所有正實數,從而就能使…
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