Mathematics
Grade-8
Easy

Question

The rule (x,  y) → ( - y,  - x) describes

  1. Reflection of point over the line y = x
  2. Reflection of point over the line y =  - x
  3. Rotation of point 90° counter clockwise
  4. Rotation of point 90° clockwise

hintHint:

A rotation is a transformation in a plane that turns every point of a preimage through a specified angle and direction about a fixed point.

The correct answer is: Reflection of point over the line y =  - x


    The rule (x, y) → ( - y, - x) describes reflection about the line y = x.
    Hence, the correct option is A.

    A point can be reflected about any of the two axes or about any line.

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    >>>From the given data:
    (x', y') = (x, y)
    * By comparing the above Equation's we get:
    x = (x cosalpha - y sinalpha) and y = y cosalpha + x sinalpha
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    ___________________________________
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    In which rotation movement does (x, y)      (-y, x)

    Given Data:
    In which rotation movement does (x, y)      (-y, x)
    >>>We were asked to find the angle of rotation of a point to rotate a point from (x, y) to (-y, x).
    *** Rotated coordinates are:
    (x', y') =  (x cosalpha - y sinalpha , y cosalpha + x sinalpha)
    >>Here, the rotated points are :
                  (x', y') = (-y, x).
    * Hence, By comparing the above equation's we get:
                 -y =  x cosalpha - y sinalpha ; and x = y cosalpha + x sinalpha

    Hence, By solving the above equation's we get:

                (x cross times -y) = x2cosalpha - (x cross times y)sinalpha

    and      (y cross times x) = y2 cosalpha + (x cross times y)sinalpha
              ________________________________
                          0 = ( x2 + y2)cosalpha 

    * Hence, cosalpha =0 leads to 90 degrees or -270 degrees.
    >>>>Therefore, the Angle of Rotation is counter clockwise 90 degrees and clockwise 270 degrees.

    In which rotation movement does (x, y)      (-y, x)

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    Given Data:
    In which rotation movement does (x, y)      (-y, x)
    >>>We were asked to find the angle of rotation of a point to rotate a point from (x, y) to (-y, x).
    *** Rotated coordinates are:
    (x', y') =  (x cosalpha - y sinalpha , y cosalpha + x sinalpha)
    >>Here, the rotated points are :
                  (x', y') = (-y, x).
    * Hence, By comparing the above equation's we get:
                 -y =  x cosalpha - y sinalpha ; and x = y cosalpha + x sinalpha

    Hence, By solving the above equation's we get:

                (x cross times -y) = x2cosalpha - (x cross times y)sinalpha

    and      (y cross times x) = y2 cosalpha + (x cross times y)sinalpha
              ________________________________
                          0 = ( x2 + y2)cosalpha 

    * Hence, cosalpha =0 leads to 90 degrees or -270 degrees.
    >>>>Therefore, the Angle of Rotation is counter clockwise 90 degrees and clockwise 270 degrees.

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    In rotation of clockwise movement maps (x , y) (y,-x)

    Given Data:
    The point (x, y) is transformed to (x , y) (y,-x) in clockwise direction.
    >>> we were asked to find the Angle of Rotation.
    >>>The coordinates of a point (x, y) after rotation through 90 degrees in clockwise direction are:
    (x', y') = (x cosalpha - y sinalpha , y cosalpha + x sinalpha)
    >>>we were given that (x', y') = (y, -x)
    >>> (y, -x) = (x cosalpha - y sinalpha , y cosalpha + x sinalpha)
        Hence, y = x cosalpha - y sinalpha and -x = y cosalpha + x sinalpha
    By solving the above equation's we get:
                  (x cross times y)  = x2cosalpha - (x cross times y) sinalpha
    and        (y cross times -x) = y2cosalpha + (x cross times y) sinalpha

    __________________________________
    0 = (x2+y2)cosalpha
    *This implies cosalpha=0, then:
    alpha = 90 degrees.
    >>>Therefore, the angle of rotation is 90 degrees.

    In rotation of clockwise movement maps (x , y) (y,-x)

    MathematicsGrade-8

    Given Data:
    The point (x, y) is transformed to (x , y) (y,-x) in clockwise direction.
    >>> we were asked to find the Angle of Rotation.
    >>>The coordinates of a point (x, y) after rotation through 90 degrees in clockwise direction are:
    (x', y') = (x cosalpha - y sinalpha , y cosalpha + x sinalpha)
    >>>we were given that (x', y') = (y, -x)
    >>> (y, -x) = (x cosalpha - y sinalpha , y cosalpha + x sinalpha)
        Hence, y = x cosalpha - y sinalpha and -x = y cosalpha + x sinalpha
    By solving the above equation's we get:
                  (x cross times y)  = x2cosalpha - (x cross times y) sinalpha
    and        (y cross times -x) = y2cosalpha + (x cross times y) sinalpha

    __________________________________
    0 = (x2+y2)cosalpha
    *This implies cosalpha=0, then:
    alpha = 90 degrees.
    >>>Therefore, the angle of rotation is 90 degrees.

    parallel

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