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删除46字节 、 2021年9月8日 (星期三)
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{{More footnotes|date=July 2013}}
 
{{More footnotes|date=July 2013}}
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[[Image:x cubed plot.svg|thumb|Plot of {{math|1=''y'' = ''x''<sup>3</sup>}} with an inflection point at (0,0), which is also a [[stationary point]].]]
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[[Image:x cubed plot.svg|thumb|Plot of {{math|1=''y'' = ''x''<sup>3</sup>}} with an inflection point at (0,0), which is also a [[stationary point]].|链接=Special:FilePath/X_cubed_plot.svg]]
    
Plot of  with an inflection point at (0,0), which is also a [[stationary point.]]
 
Plot of  with an inflection point at (0,0), which is also a [[stationary point.]]
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In [[differential calculus]] and differential geometry, an '''inflection point''', '''point of inflection''', '''flex''', or '''inflection''' (British English: '''inflexion''') is a point on a [[plane curve#Smooth plane curve|smooth plane curve]] at which the [[signed curvature|curvature]] changes sign. In particular, in the case of the [[graph of a function]], it is a point where the function changes from being [[Concave function|concave]] (concave downward) to [[convex function|convex]] (concave upward), or vice versa.
 
In [[differential calculus]] and differential geometry, an '''inflection point''', '''point of inflection''', '''flex''', or '''inflection''' (British English: '''inflexion''') is a point on a [[plane curve#Smooth plane curve|smooth plane curve]] at which the [[signed curvature|curvature]] changes sign. In particular, in the case of the [[graph of a function]], it is a point where the function changes from being [[Concave function|concave]] (concave downward) to [[convex function|convex]] (concave upward), or vice versa.
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In differential calculus and differential geometry, an inflection point, point of inflection, flex, or inflection (British English: inflexion) is a point on a smooth plane curve at which the curvature changes sign. In particular, in the case of the graph of a function, it is a point where the function changes from being concave (concave downward) to convex (concave upward), or vice versa.
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在微分和微分几何中,拐点(英文名为inflection point,point of infection,,flex,或者inflection,英式拼写为inflextion)是光滑平面曲线上的曲率符号改变的点。在函数图像中,拐点处函数从下凹变为上凸 ,或从上凸变为下凹。
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在微分和微分几何中,拐点是光滑平面曲线上的改变曲率符号的一个点。特别地,在函数图像中,拐点处函数从凹(向下)变为凸(向上) ,反之亦然。
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For example, if the curve is the graph of a function {{math|1=''y'' = ''f''(''x'')}}, of [[differentiability class]] {{math|''C''<sup>2</sup>}}, an inflection point of the curve is where ''f<nowiki>''</nowiki>'', the [[second derivative]] of {{mvar|f}}, vanishes (''f<nowiki>''</nowiki> = 0'') and changes its sign at the point (from positive to negative or from negative to positive).<ref>{{Cite book|last=Stewart|first=James|title=Calculus|publisher=Cengage Learning|year=2015|isbn=978-1-285-74062-1|edition=8|location=Boston|pages=281}}</ref> A point where the second derivative vanishes but does not change its sign is sometimes called a '''point of undulation''' or '''undulation point'''.
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For example, if the curve is the graph of a function {{math|1=''y'' = ''f''(''x'')}}, of [[differentiability class]] {{math|''C''<sup>2</sup>}}, an inflection point of the curve is where ''f<nowiki>''</nowiki>'', the [[second derivative]] of {{mvar|f}}, vanishes (''f<nowiki>''</nowiki> = 0'') and changes its sign at the point (from positive to negative or from negative to positive).<ref name=":0">{{Cite book|last=Stewart|first=James|title=Calculus|publisher=Cengage Learning|year=2015|isbn=978-1-285-74062-1|edition=8|location=Boston|pages=281}}</ref> A point where the second derivative vanishes but does not change its sign is sometimes called a '''point of undulation''' or '''undulation point'''.
    
For example, if the curve is the graph of a function , of differentiability class , an inflection point of the curve is where f<nowiki></nowiki>, the second derivative of , vanishes (f<nowiki></nowiki> = 0) and changes its sign at the point (from positive to negative or from negative to positive). A point where the second derivative vanishes but does not change its sign is sometimes called a point of undulation or undulation point.
 
For example, if the curve is the graph of a function , of differentiability class , an inflection point of the curve is where f<nowiki></nowiki>, the second derivative of , vanishes (f<nowiki></nowiki> = 0) and changes its sign at the point (from positive to negative or from negative to positive). A point where the second derivative vanishes but does not change its sign is sometimes called a point of undulation or undulation point.
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例如,若曲线是可导性类的函数图像,那么在曲线拐点处二阶导数为0,并且改变了它的符号(从正到负或从负到正)。二阶导数为0但其符号不变的点有时称为波动点。
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例如,若曲线{{math|1=''y'' = ''f''(''x'')}}有二阶导数,那么拐点处曲线二阶导数''f<nowiki>'''</nowiki>''为0(f<nowiki>''</nowiki>=0),并且符号改变(从正到负或从负到正)<ref name=":0" />。二阶导数为0但其符号不变的点有时称为波动点。
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[[Image:Animated illustration of inflection point.gif|upright=2.5|thumb|Plot of {{math|''f''(''x'') {{=}} sin(2''x'')}} from −{{pi}}/4 to 5{{pi}}/4; the second [[derivative]] is {{math|''f{{''}}''(''x'') {{=}} –4sin(2''x'')}}, and its sign is thus the opposite of the sign of {{mvar|f}}. Tangent is blue where the curve is [[convex function|convex]] (above its own [[tangent line|tangent]]), green where concave (below its tangent), and red at inflection points: 0, {{pi}}/2 and {{pi}}]]
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[[Image:Animated illustration of inflection point.gif|upright=2.5|thumb|Plot of {{math|''f''(''x'') {{=}} sin(2''x'')}} from −{{pi}}/4 to 5{{pi}}/4; the second [[derivative]] is {{math|''f{{''}}''(''x'') {{=}} –4sin(2''x'')}}, and its sign is thus the opposite of the sign of {{mvar|f}}. Tangent is blue where the curve is [[convex function|convex]] (above its own [[tangent line|tangent]]), green where concave (below its tangent), and red at inflection points: 0, {{pi}}/2 and {{pi}}|链接=Special:FilePath/Animated_illustration_of_inflection_point.gif]]
    
Plot of  sin(2x)}} from −/4 to 5/4; the second [[derivative is (x)  –4sin(2x)}}, and its sign is thus the opposite of the sign of . Tangent is blue where the curve is convex (above its own tangent), green where concave (below its tangent), and red at inflection points: 0, /2 and ]]
 
Plot of  sin(2x)}} from −/4 to 5/4; the second [[derivative is (x)  –4sin(2x)}}, and its sign is thus the opposite of the sign of . Tangent is blue where the curve is convex (above its own tangent), green where concave (below its tangent), and red at inflection points: 0, /2 and ]]
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If the second derivative, (x)}} exists at , and  is an inflection point for , then (x<sub>0</sub>)  0}}, but this condition is not sufficient for having a point of inflection, even if derivatives of any order exist. In this case, one also needs the lowest-order (above the second) non-zero derivative to be of odd order (third, fifth, etc.). If the lowest-order non-zero derivative is of even order, the point is not a point of inflection, but an undulation point. However, in algebraic geometry, both inflection points and undulation points are usually called inflection points. An example of an undulation point is  0}} for the function  given by  x<sup>4</sup>}}.
 
If the second derivative, (x)}} exists at , and  is an inflection point for , then (x<sub>0</sub>)  0}}, but this condition is not sufficient for having a point of inflection, even if derivatives of any order exist. In this case, one also needs the lowest-order (above the second) non-zero derivative to be of odd order (third, fifth, etc.). If the lowest-order non-zero derivative is of even order, the point is not a point of inflection, but an undulation point. However, in algebraic geometry, both inflection points and undulation points are usually called inflection points. An example of an undulation point is  0}} for the function  given by  x<sup>4</sup>}}.
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如果二阶导数,(x)}}在x0处存在,并且x0是该函数的拐点,那么(x < sub > 0 </sub >)0} ,那么即使存在任意阶的导数,这个条件对于有拐点也是不充分的。在这种情况下,还需要最低阶(第二阶以上)非零导数为奇数阶(第三阶、第五阶等)。若最低阶非零导数为偶数阶,则该点不是拐点,而是波动点。然而,在代数几何中,拐点和起伏点被统称为拐点。对于给定的 x < sup > 4 </sup > }的函数,波动点是0}}。
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<nowiki>如果二阶导数,(x)}}在x0处存在,并且x0是该函数的拐点,那么(x < sub > 0 )0} ,那么即使存在任意阶的导数,这个条件对于有拐点也是不充分的。在这种情况下,还需要最低阶(第二阶以上)非零导数为奇数阶(第三阶、第五阶等)。若最低阶非零导数为偶数阶,则该点不是拐点,而是波动点。然而,在代数几何中,拐点和起伏点被统称为拐点。对于给定的 x < sup > 4 }的函数,波动点是0}}。</nowiki>
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拐点的分类
 
拐点的分类
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[[Image:X to the 4th minus x.svg|thumb|upright=1.2|{{math|''y'' {{=}} ''x''<sup>4</sup> – ''x''}} has a 2nd derivative of zero at point (0,0), but it is not an inflection point because the fourth derivative is the first higher order non-zero derivative (the third derivative is zero as well).]]
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[[Image:X to the 4th minus x.svg|thumb|upright=1.2|{{math|''y'' {{=}} ''x''<sup>4</sup> – ''x''}} has a 2nd derivative of zero at point (0,0), but it is not an inflection point because the fourth derivative is the first higher order non-zero derivative (the third derivative is zero as well).|链接=Special:FilePath/X_to_the_4th_minus_x.svg]]
    
  x<sup>4</sup> – x}} has a 2nd derivative of zero at point (0,0), but it is not an inflection point because the fourth derivative is the first higher order non-zero derivative (the third derivative is zero as well).
 
  x<sup>4</sup> – x}} has a 2nd derivative of zero at point (0,0), but it is not an inflection point because the fourth derivative is the first higher order non-zero derivative (the third derivative is zero as well).
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