Lösung 1.1:5
Aus Online Mathematik Brückenkurs 2
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- | Suppose that the tangent touches the curve at the point | + | Suppose that the tangent touches the curve at the point <math>(x_0,y_0)</math>. That point must, first and foremost, lie on the curve and therefore satisfy the equation of the curve, i.e. |
- | <math> | + | |
+ | {{Displayed math||<math>y_0 = -x_0^2\,\textrm{.}</math>|(1)}} | ||
- | <math> | + | If we now write the equation of the tangent as <math>y=kx+m</math>, the slope of the tangent, ''k'', is given by the value of the curve's derivative, <math>y^{\,\prime} = -2x</math>, at <math>x=x_0</math>, |
+ | {{Displayed math||<math>k = -2x_0\,\textrm{.}</math>|(2)}} | ||
- | + | The condition that the tangent goes through the point <math>(x_0,y_0)</math> gives us that | |
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- | <math> | + | |
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+ | {{Displayed math||<math>y_{0} = k\cdot x_0 + m\,\textrm{.}</math>|(3)}} | ||
- | + | In addition to this, the tangent should also pass through the point (1,1), | |
- | + | {{Displayed math||<math>1 = k\cdot 1 + m\,\textrm{.}</math>|(4)}} | |
- | <math>\ | + | |
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+ | Equations (1)-(4) constitute a system of equations in the unknowns <math>x_0</math>, <math>y_{0}</math>, <math>k</math> and <math>m</math>. | ||
- | <math> | + | Because we are looking for <math>x_0</math> and <math>y_0</math>, the first step is to try and eliminate ''k'' and ''m'' from the equations. |
+ | Equation (2) gives that <math>k = -2 x_0</math> and substituting this into equation (4) gives | ||
- | + | {{Displayed math||<math>1 = -2x_0 + m\quad\Leftrightarrow\quad m = 2x_0+1\,\textrm{.}</math>}} | |
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+ | With ''k'' and ''m'' expressed in terms of <math>x_0</math> and <math>y_0</math>, (3) becomes an equation that is expressed completely in terms of <math>x_0</math> | ||
+ | and <math>y_0</math>, | ||
- | <math> | + | {{Displayed math||<math>y_0 = -2x_0^2 + 2x_0 + 1\,\textrm{.}</math>|(3')}} |
+ | This equation, together with (1), is a system of equations in <math>x_0</math> and <math>y_0</math>, | ||
- | + | {{Displayed math||<math>\left\{\begin{align} | |
- | <math> | + | y_{0} &= -x_0^{2}\,,\\[5pt] |
- | + | y_{0} &= -2x_0^2 + 2x_0 + 1\,\textrm{.} | |
- | + | \end{align}\right.</math>}} | |
- | + | Substituting equation (1) into (3') gives us an equation in <math>x_0</math>, | |
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- | + | {{Displayed math||<math>-x_0^2 = -2x_0^2 + 2x_0 + 1\,,</math>}} | |
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i.e. | i.e. | ||
+ | {{Displayed math||<math>x_0^2 - 2x_0 - 1 = 0\,\textrm{.}</math>}} | ||
- | + | This quadratic equation has solutions | |
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- | This | + | |
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- | + | {{Displayed math||<math>x_0 = 1-\sqrt{2}\qquad\text{and}\qquad x_0 = 1+\sqrt{2}\,\textrm{.}</math>}} | |
- | <math> | + | |
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+ | Equation (1) gives the corresponding y-values, | ||
- | + | {{Displayed math||<math>y_0 = -3+2\sqrt{2}\qquad\text{and}\qquad y_0 = -3-2\sqrt{2}\,\textrm{.}</math>}} | |
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+ | Thus, the answers are the points <math>(1-\sqrt{2},-3+2\sqrt{2})</math> and | ||
+ | <math>(1+\sqrt{2},-3-2\sqrt{2})\,</math>. | ||
[[Image:1_1_5_3.gif|center]] | [[Image:1_1_5_3.gif|center]] |
Version vom 13:32, 14. Okt. 2008
Suppose that the tangent touches the curve at the point y0)
(1) |
If we now write the equation of the tangent as =−2x
(2) |
The condition that the tangent goes through the point y0)
![]() | (3) |
In addition to this, the tangent should also pass through the point (1,1),
![]() | (4) |
Equations (1)-(4) constitute a system of equations in the unknowns
Because we are looking for
Equation (2) gives that
![]() |
With k and m expressed in terms of
(3') |
This equation, together with (1), is a system of equations in
![]() ![]() ![]() ![]() |
Substituting equation (1) into (3') gives us an equation in
![]() |
i.e.
This quadratic equation has solutions
![]() ![]() |
Equation (1) gives the corresponding y-values,
![]() ![]() |
Thus, the answers are the points 2
−3+2
2)
2
−3−2
2)