二次函數(shù)
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的圖象如圖所示,
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是圖象上的一點,且
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,則
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的值為:
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A.-2 | B.-1 | C. | D. |
設ax
2+bx+c=0的兩根分別為x
1與x
2,依題意有AQ
2+BQ
2=AB
2.(x
1-n)
2+4+(x
2-n)
2+4=(x
1-x
2)
2,化簡得:n
2-n(x
1+x
2)+4+x
1x
2=0.有n
2+
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n+4+
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=0,∴an
2+bn+c=-4a.∵(n,2)是圖象上的一點,∴an
2+bn+c=2,∴-4a=2,故可知a=
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選D.
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.
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
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
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
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
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
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
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