求中心在 (0,4)、焦点在 (8,4)、顶点在 (6, 7) 的椭圆的方程
圆锥截面,通常称为圆锥曲线,是在平面与圆锥相交时创建的。这些部分相交的角度决定了它们的几何形状。因此,圆锥截面分为四种类型:圆、椭圆、抛物线和双曲线。这些类型中的每一种都以其独特的数学属性和方程式集而著称。椭圆将在下面详细解释。
椭圆
椭圆是当平面以小于直角但大于圆锥顶点所成角度 (α) 的角度 (β) 与圆锥相交时生成的圆锥截面。换句话说,当平面以角度 β 切割圆锥体时生成椭圆,使得 α<β<90 o 。
如上图所示,锥体和平面以小于直角但大于在锥体顶点处形成的角度的角度相交,从而由于相交而形成椭圆。
椭圆方程
- 以 (h, k) 为中心且长轴平行于 x 轴的椭圆的标准方程由下式给出:
,
where the coordinates of the vertex are (h±a, 0), coordinates of co-vertex are (h, k±b) and the coordinates of foci are (h±c, k), where c2 = a2 – b2.
- 以 (h, k) 为中心且长轴平行于 y 轴的椭圆的标准方程由下式给出:
,
where the coordinates of the vertex are (h, k±a), coordinates of co-vertex are (h±b, k) and the coordinates of foci are (h, k±c), where c2 = a2 – b2.
找到中心在 (0,4)、焦点在 (8,4)、顶点在 (6, 7) 的椭圆的方程。
解决方案:
To find the equation of an ellipse, we need the values a and b.
The distance between the center (0, 4) and the focus (8, 4) gives c.
So, c2 = (8 – 0)2 + (4 – 4)2
c2 = 64
The distance between the center (0, 4) and the vertex (6, 7) gives a.
So, a2 = (6 – 0)2 + (7 – 4)2
a2 = 36 + 9
a2 = 45
Put the obtained values in the formula c2 = a2 – b2 to find b.
b2 = 64 – 45
b2 = 19
As the y-coordinates of center and focus are same, ellipse lies on x-axis. So the equation is of the form
Hence the required equation of the ellipse is,
类似问题
问题 1. 找出位于 y 轴上、中心在 (0,7)、长轴长度为 12 个单位、短轴长度为 16 个单位的椭圆的方程。
解决方案:
The length of major axis is, 2a = 12 => a = 6.
The length of minor axis is, 2b = 16 => b = 8.
As the ellipse lies on y-axis, the equation is
So, the equation is .
问题 2. 求一个椭圆方程,它位于 x 轴上,中心在 (3,2),长轴长度为 2 个单位,短轴长度为 8 个单位。
解决方案:
The length of major axis is, 2a = 2 => a = 1.
The length of minor axis is, 2b = 8 => b = 4.
As the ellipse lies on x-axis, the equation is
So, the equation is .
问题 3. 用焦点 (0, 6) 和短轴 (8, 0) 找到椭圆长轴的坐标。
解决方案:
We have, c = 6 and b = 8.
Put these in c2 = a2 – b2 to find a.
a2 = 62 + 82
a2 = 100
a = 10
The coordinates of major axis are (0, ±10).
问题 4. 找出以 (7, 2) 为中心且 c = 3 且 b = 4 的椭圆的方程。
解决方案:
Given b = 4, c = 3, h = 7 and k = 2.
Put these in the formula c2 = a2 – b2 to find a.
a2 = 42 + 32
a2 = 25
a = 5
As the ellipse lies on x-axis, the equation is of the form
So, the equation is,
问题 5. 如果 a = 12,b = 6,并且短轴平行于 x 轴,则求以原点为中心的椭圆方程。
解决方案:
The minor axis is parallel to x-axis, so the ellipse lies on y-axis.
The equation is of the form .
Here a = 12 and b = 6, h = 0 and k = 0.
So, the equation becomes,