Subsection 12.3

Coordinate representation of linear and affine transformations of the plane. In the problems, the coordinate system is assumed to be a general Cartesian one. (12.37-12.52)

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Problem 12.37

Write the formulas defining the given transformation of the plane:

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(1)

a homothety with center at the origin and ratio kk

(2)

a homothety with center at the point M(x0,y0)M\left(x_{0}, y_{0}\right) and ratio kk

(3)

a point reflection with respect to the point M(x0,y0)M\left(x_{0}, y_{0}\right)

(4)

a translation by the vector a(α,β)\mathbf{a}(\alpha , \beta ).

Problem 12.38

An affine transformation of the plane is given by the formulas x∗=3x+2y−6,y∗=4x−3y+1x^{ * } = 3x + 2y - 6, \quad y^{ * } = 4x - 3y + 1. Find the images:

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(1)

of the points

(1) O(0,0)O(0, 0);

(2) E1(1,0)E_{1}(1, 0);

(3) E2(0,1)E_{2}(0, 1);

(4) E(1,1)E(1, 1);

(5) M(−1,5)M(- 1, 5);

(2)

of the lines

(1) y=0y = 0;

(2) x=0x = 0;

(3) x−y+1=0x - y + 1 = 0;

(4) x−y−1=0x - y - 1 = 0;

(5) 2x+3y=72x + 3y = 7.

Problem 12.39

An affine transformation of the plane is given by the formulas x∗=2x+3y−1,y∗=−3x−4y+2x^{ * } = 2x + 3y - 1, y^{ * } = - 3x - 4y + 2. Find the preimages:

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(1)

of the points

(1) O(0,0)O(0, 0);

(2) A(−1,2)A(- 1, 2);

(3) B(4,−5)B(4, - 5)

(2)

of the lines

(1) y=0y = 0;

(2) x=0x = 0;

(3) x+y−1=0x + y - 1 = 0;

(4) x−y−1=0x - y - 1 = 0;

(5) x−y+1=0x - y + 1 = 0.

Problem 12.40

Write the formulas defining the affine transformation of the plane taking the points A,B,CA, B, C respectively to A∗,B∗,C∗A^{ * }, B^{ * }, C^{ * }:

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(1)

A(1,0),B(0,1),C(1,1),A∗(−3,5),B∗(4,−3),C∗(0,0)A(1, 0), B(0, 1), C(1, 1), A^{ * }(- 3, 5), B^{ * }(4, - 3), C^{ * }(0, 0)

(2)

A(3/7,1),B(1,1/4),C(2,−1),A∗(−4,2),B∗(−1,6)A(3 / 7, 1), \quad B(1, 1 / 4), \quad C(2, - 1), \quad A^{ * }(- 4, 2), \quad B^{ * }(- 1, 6), C∗(4,13)C^{ * }(4, 13)

(3)

A(1,0),B(−1/2,3/2),C(−1/2,−3/2),A∗=BA(1, 0), \quad B(- 1 / 2, \sqrt{3} / 2), \quad C(- 1 / 2, - \sqrt{3} / 2), \quad A^{ * } = B, B∗=C,C∗=AB^{ * } = C, C^{ * } = A

(4)

A(1,2),B(−7,4),C(3,−6);A∗,B∗,C∗A(1, 2), B(- 7, 4), C(3, - 6); A^{ * }, B^{ * }, C^{ * } are the midpoints of the sides of the triangle ABCABC opposite the vertices A,B,CA, B, C respectively.

Problem 12.41

Find all possible linear transformations of the plane taking the points A,B,CA, B, C respectively to A∗A^{ * }, B∗,C∗B^{ * }, C^{ * }, if such transformations exist:

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(1)

A(1,4),B(−2,1),C(0,3),A∗(0,0),B∗(1,0),C∗(0,4)A(1, 4), B(- 2, 1), C(0, 3), A^{ * }(0, 0), B^{ * }(1, 0), C^{ * }(0, 4)

(2)

A(−2,0),B(2,−1),C(0,4),A∗(−2,1),B∗(2,1),C∗(0,1)A(- 2, 0), B(2, - 1), C(0, 4), A^{ * }(- 2, 1), B^{ * }(2, 1), C^{ * }(0, 1)

(3)

A(2,0),B(3,−1),C(4,−2),A∗(2,1),B∗(−2,−1)A(2, 0), \quad B(3, - 1), \quad C(4, - 2), \quad A^{ * }(2, 1), \quad B^{ * }(- 2, - 1), C∗(−6,−3)C^{ * }(- 6, - 3)

(4)

A(0,0),B(−1,2),C(1,−2),A∗(−1,−1),B∗(0,0)A(0, 0), \quad B(- 1, 2), \quad C(1, - 2), \quad A^{ * }(- 1, - 1), \quad B^{ * }(0, 0), C∗(1,1)C^{ * }(1, 1)

Problem 12.42

Find all fixed points of the affine transformation given by the formulas:

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(1)

x∗=7x−3y,y∗=x+yx^{ * } = 7x - 3y, \quad y^{ * } = x + y

(2)

x∗=−5x+y,y∗=6xx^{ * } = - 5x + y, \quad y^{ * } = 6x

(3)

x∗=−5x+y,y∗=6x+1x^{ * } = - 5x + y, \quad y^{ * } = 6x + 1

(4)

x∗=2x−y+3,y∗=−2x+2y−6x^{ * } = 2x - y + 3, \quad y^{ * } = - 2x + 2y - 6

(5)

x∗=4x+3y−1,y∗=−3x−2y+1x^{ * } = 4x + 3y - 1, \quad y^{ * } = - 3x - 2y + 1

(6)

x∗=x,y∗=yx^{ * } = x, \quad y^{ * } = y.

Problem 12.43

Find the invariant lines of the linear transformation given by the formulas:

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(1)

x∗=2x+3y,y∗=−yx^{ * } = 2x + 3y, \quad y^{ * } = - y

(2)

x∗=−x+y,y∗=x−yx^{ * } = - x + y, \quad y^{ * } = x - y

(3)

x∗=y−9,y∗=9x+1x^{ * } = y - 9, \quad y^{ * } = 9x + 1

(4)

x∗=y,y∗=−x+1x^{ * } = y, \quad y^{ * } = - x + 1

(5)

x∗=2x+y−3,y∗=2x+3y−6x^{ * } = 2x + y - 3, \quad y^{ * } = 2x + 3y - 6

(6)

x∗=5x+3y+1,y∗=−3x−yx^{ * } = 5x + 3y + 1, \quad y^{ * } = - 3x - y

(7)

x∗=3x−2y+5,y∗=2x−y+5x^{ * } = 3x - 2y + 5, \quad y^{ * } = 2x - y + 5.

Problem 12.44

Prove that the determinant △=[a1b1a2b2]\triangle =\left[\begin{smallmatrix} a_{1} & b_{1} \\ a_{2} & b_{2}\end{smallmatrix}\right] of the linear transformation given by the formulas

x∗=a1x+b1y+c1,y∗=a2x+b2y+c2 x^{ * } = a_{1} x + b_{1} y + c_{1}, \quad y^{ * } = a_{2} x + b_{2} y + c_{2}

does not depend on the choice of coordinate system.

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Problem 12.45

Points A,B,CA, B, C have coordinates (1,0),(0,1),(1,1)(1, 0), (0, 1), (1, 1) respectively in the coordinate system OO, e1\mathbf{e}_{1}, e2\mathbf{e}_{2}, and coordinates (1,−1),(−3,2),(0,1)(1, - 1), (- 3, 2), (0, 1) respectively in the coordinate system O∗,e1∗,e2∗O^{ * }, \mathbf{e}_{1}^{ * }, \mathbf{e}_{2}^{ * }. Write the formulas defining, in the coordinate system O,e1,e2O, \mathbf{e}_{1}, \mathbf{e}_{2}, the affine transformation ff such that f(O)=O∗f(O) = O^{ * }, f(e1)=e1∗,f(e2)=e2∗f\left(\mathbf{e}_{1}\right) = \mathbf{e}_{1}^{ * }, f\left(\mathbf{e}_{2}\right) = \mathbf{e}_{2}^{ * }.

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Problem 12.46

The formulas for the passage from the coordinate system OO, e1,e2\mathbf{e}_{1}, \mathbf{e}_{2} to the system O′,e1′,e2′O^{\prime }, \mathbf{e}_{1}^{\prime }, \mathbf{e}_{2}^{\prime } are given. Write the formulas defining, in the coordinate system O,e1,e2O, \mathbf{e}_{1}, \mathbf{e}_{2}, the affine transformation ff such that f(O)=O′,f(e1)=e1′,f(e2)=e2′f(O) = O^{\prime }, \quad f\left(\mathbf{e}_{1}\right) = \mathbf{e}_{1}^{\prime }, \quad f\left(\mathbf{e}_{2}\right) = \mathbf{e}_{2}^{\prime }:

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(1)

x=x′+y′−2,y=2x′−y′+3x = x^{\prime } + y^{\prime } - 2, \quad y = 2x^{\prime } - y^{\prime } + 3

(2)

x=3x′−4y′−5,y=4x′+3y′+1x = 3x^{\prime } - 4y^{\prime } - 5, \quad y = 4x^{\prime } + 3y^{\prime } + 1.

Problem 12.47

Write the formulas defining the affine transformation:

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(1)

taking the lines x−y+1=0,x+y−1=0x - y + 1 = 0, \quad x + y - 1 = 0 respectively to the lines 3x+2y−3=0,2x+3y+1=03x + 2y - 3 = 0, 2x + 3y + 1 = 0, and the point A(1,1)A(1, 1) to the point B(−1,−2)B(- 1, - 2)

(2)

taking the lines A1x+B1y+C1=0A_{1} x + B_{1} y + C_{1} = 0 and A2x+B2y+C2=0A_{2} x + B_{2} y + C_{2} = 0 respectively to the yy-axis and the xx-axis, and the point A(x0,y0)A\left(x_{0}, y_{0}\right) to the point B(1,1)B(1, 1) (the point A\mathrm{A} does not lie on the given lines).

Problem 12.48

Given the affine transformation x∗=2x+3y,y∗=3x+5yx^{ * } = 2x + 3y, y^{ * } = 3x + 5y. Find the equation of the image of the curve:

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(1)

x2+y2=1x^{2} + y^{2} = 1

(2)

x2−y2=1x^{2} - y^{2} = 1

(3)

xy=2xy = 2

(4)

y2=−6xy^{2} = - 6x

(5)

(3x+4y−1)(4x−3y+1)=0(3x + 4y - 1)(4x - 3y + 1) = 0

(6)

(x+y−1)(x+y+1)=2(x + y - 1)(x + y + 1) = 2.

Problem 12.49

Given the affine transformation x∗=−x+5y+1x^{ * } = - x + 5y + 1, y∗=3x−2y−1y^{ * } = 3x - 2y - 1. Find the equations of the preimage of the curve:

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(1)

x2+y2=4x^{2} + y^{2} = 4

(2)

x24−y2=1\frac{x^{2}}{4} - y^{2} = 1

(3)

(y+1)2=8(x−1)(y + 1)^{2} = 8(x - 1)

(4)

(x+y−1)(x−y−1)=1(x + y - 1)(x - y - 1) = 1

(5)

(x+2y−2)(x+2y+2)=0(x + 2y - 2)(x + 2y + 2) = 0.

Problem 12.50
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(1)

Write the formulas of an affine transformation of the first kind taking the ellipse x24+y2=1\frac{x^{2}}{4} + y^{2} = 1 to itself so that the point A(1,3/2)A(1, \sqrt{3}/2) goes to the point B(−2,0)B(- 2, 0).

(2)

Solve the same problem for an affine transformation of the second kind.

Problem 12.51

Write the formulas of an affine transformation taking the hyperbola x25−y24=1\frac{x^{2}}{5} - \frac{y^{2}}{4} = 1 to itself so that the point A(5,4)A(5, 4) goes to the point B(5,0)B(\sqrt{5}, 0).

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Problem 12.52

Find the affine transformation, if it takes the parabola x2=4yx^{2} = 4y to itself and:

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(1)

the point A1(2,1)A_{1}(2, 1) goes to the point B1(4,4)B_{1}(4, 4), and the point A2(1A_{2}(1, 1/4)1 / 4) goes to the point B2(3,9/4)B_{2}(3, 9 / 4)

(2)

the determinant of the transformation equals 1.