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Solve the equations for x and y. Find the eigenvalues and eigenvectors for the given matrix A.

Mathematics

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here are the solutions to questions 8a and 8b.

8. a) Solve the equations for xx and yy.
Given the equation:

(x−i2y)+(y−i3x)=2+i3(x - i2y) + (y - i3x) = 2 + i3
  1. Group the real and imaginary parts on the left side of the equation.

    The real parts are xx and yy.
    The imaginary parts are −2y-2y (from −i2y-i2y) and −3x-3x (from −i3x-i3x).
    So, the equation becomes:

    (x+y)+i(−2y−3x)=2+i3(x+y) + i(-2y-3x) = 2 + i3
  2. Equate the real parts and the imaginary parts from both sides of the equation.

    Equating the real parts:

    x+y=2(Equation 1)x + y = 2 \quad \text{(Equation 1)}

    Equating the imaginary parts:

    −2y−3x=3(Equation 2)-2y - 3x = 3 \quad \text{(Equation 2)}
  3. Solve the system of linear equations.

    From Equation 1, express yy in terms of xx:

    y=2−xy = 2 - x

    Substitute this expression for yy into Equation 2:

    −2(2−x)−3x=3-2(2 - x) - 3x = 3 −4+2x−3x=3-4 + 2x - 3x = 3 −4−x=3-4 - x = 3 −x=3+4-x = 3 + 4 −x=7-x = 7 x=−7x = -7

    Now substitute the value of xx back into the expression for yy:

    y=2−(−7)y = 2 - (-7) y=2+7y = 2 + 7 y=9y = 9

    The solutions are x=−7,y=9\boxed{x = -7, y = 9}.

    8. b) Find the eigenvalues and eigenvectors for the given matrix A.
    Given the matrix:

    A=[22−2132−111]A = \begin{bmatrix} 2 & 2 & -2 \\ 1 & 3 & 2 \\ -1 & 1 & 1 \end{bmatrix}
  4. Find the eigenvalues by solving the characteristic equation det⁡(A−λI)=0\det(A - \lambda I) = 0.

    A−λI=[2−λ2−213−λ2−111−λ]A - \lambda I = \begin{bmatrix} 2-\lambda & 2 & -2 \\ 1 & 3-\lambda & 2 \\ -1 & 1 & 1-\lambda \end{bmatrix}

    Calculate the determinant:

    det⁡(A−λI)=(2−λ)[(3−λ)(1−λ)−(2)(1)]−2[(1)(1−λ)−(2)(−1)]+(−2)[(1)(1)−(3−λ)(−1)]\det(A - \lambda I) = (2-\lambda)[(3-\lambda)(1-\lambda) - (2)(1)] - 2[(1)(1-\lambda) - (2)(-1)] + (-2)[(1)(1) - (3-\lambda)(-1)] =(2−λ)[λ2−4λ+3−2]−2[1−λ+2]−2[1+3−λ]= (2-\lambda)[\lambda^2 - 4\lambda + 3 - 2] - 2[1-\lambda+2] - 2[1+3-\lambda] =(2−λ)[λ2−4λ+1]−2[3−λ]−2[4−λ]= (2-\lambda)[\lambda^2 - 4\lambda + 1] - 2[3-\lambda] - 2[4-\lambda] =(2λ2−8λ+2−λ3+4λ2−λ)−(6−2λ)−(8−2λ)= (2\lambda^2 - 8\lambda + 2 - \lambda^3 + 4\lambda^2 - \lambda) - (6 - 2\lambda) - (8 - 2\lambda) =−λ3+6λ2−5λ−12= -\lambda^3 + 6\lambda^2 - 5\lambda - 12

    Set the determinant to zero:

    −λ3+6λ2−5λ−12=0-\lambda^3 + 6\lambda^2 - 5\lambda - 12 = 0 λ3−6λ2+5λ+12=0\lambda^3 - 6\lambda^2 + 5\lambda + 12 = 0

    By testing integer factors of 12, we find that λ=−1\lambda = -1 is a root:
    (−1)3−6(−1)2+5(−1)+12=−1−6−5+12=0(-1)^3 - 6(-1)^2 + 5(-1) + 12 = -1 - 6 - 5 + 12 = 0.
    So, (λ+1)(\lambda+1) is a factor. Dividing the polynomial by (λ+1)(\lambda+1) gives λ2−7λ+12\lambda^2 - 7\lambda + 12.
    Factoring the quadratic equation λ2−7λ+12=0\lambda^2 - 7\lambda + 12 = 0:

    (λ−3)(λ−4)=0(\lambda-3)(\lambda-4) = 0

    The eigenvalues are λ1=−1,λ2=3,λ3=4\boxed{\lambda_1 = -1, \lambda_2 = 3, \lambda_3 = 4}.

  5. Find the eigenvectors for each eigenvalue.

    For λ1=−1\lambda_1 = -1:
    Solve (A−(−1)I)v=0  ⟹  (A+I)v=0(A - (-1)I)\mathbf{v} = \mathbf{0} \implies (A+I)\mathbf{v} = \mathbf{0}:

    [32−2142−112][v1v2v3]=[000]\begin{bmatrix} 3 & 2 & -2 \\ 1 & 4 & 2 \\ -1 & 1 & 2 \end{bmatrix} \begin{bmatrix} v_1 \\ v_2 \\ v_3 \end{bmatrix} = \begin{bmatrix} 0 \\ 0 \\ 0 \end{bmatrix}

    From the second and third rows, we can perform row operations:
    R2→3R2−R1R_2 \to 3R_2 - R_1: 3(1)v1+3(4)v2+3(2)v3−(3v1+2v2−2v3)=0  ⟹  10v2+8v3=0  ⟹  5v2+4v3=03(1)v_1 + 3(4)v_2 + 3(2)v_3 - (3v_1 + 2v_2 - 2v_3) = 0 \implies 10v_2 + 8v_3 = 0 \implies 5v_2 + 4v_3 = 0.
    R3→3R3+R1R_3 \to 3R_3 + R_1: 3(−1)v1+3(1)v2+3(2)v3+(3v1+2v2−2v3)=0  ⟹  5v2+4v3=03(-1)v_1 + 3(1)v_2 + 3(2)v_3 + (3v_1 + 2v_2 - 2v_3) = 0 \implies 5v_2 + 4v_3 = 0.
    Let v2=4kv_2 = 4k. Then 5(4k)+4v3=0  ⟹  20k+4v3=0  ⟹  v3=−5k5(4k) + 4v_3 = 0 \implies 20k + 4v_3 = 0 \implies v_3 = -5k.
    Substitute v2v_2 and v3v_3 into the first equation: 3v1+2(4k)−2(−5k)=0  ⟹  3v1+8k+10k=0  ⟹  3v1+18k=0  ⟹  v1=−6k3v_1 + 2(4k) - 2(-5k) = 0 \implies 3v_1 + 8k + 10k = 0 \implies 3v_1 + 18k = 0 \implies v_1 = -6k.
    Choosing k=1k=1, the eigenvector is v1=[−64−5]\boxed{\mathbf{v}_1 = \begin{bmatrix} -6 \\ 4 \\ -5 \end{bmatrix}}.

    For λ2=3\lambda_2 = 3:
    Solve (A−3I)v=0(A - 3I)\mathbf{v} = \mathbf{0}:

    [−12−2102−11−2][v1v2v3]=[000]\begin{bmatrix} -1 & 2 & -2 \\ 1 & 0 & 2 \\ -1 & 1 & -2 \end{bmatrix} \begin{bmatrix} v_1 \\ v_2 \\ v_3 \end{bmatrix} = \begin{bmatrix} 0 \\ 0 \\ 0 \end{bmatrix}

    From the second equation: v1+2v3=0  ⟹  v1=−2v3v_1 + 2v_3 = 0 \implies v_1 = -2v_3.
    Substitute v1v_1 into the first equation: −(−2v3)+2v2−2v3=0  ⟹  2v3+2v2−2v3=0  ⟹  2v2=0  ⟹  v2=0-(-2v_3) + 2v_2 - 2v_3 = 0 \implies 2v_3 + 2v_2 - 2v_3 = 0 \implies 2v_2 = 0 \implies v_2 = 0.
    Let v3=kv_3 = k. Then v1=−2kv_1 = -2k and v2=0v_2 = 0.
    Choosing k=1k=1, the eigenvector is v2=[−201]\boxed{\mathbf{v}_2 = \begin{bmatrix} -2 \\ 0 \\ 1 \end{bmatrix}}.

    For λ3=4\lambda_3 = 4:
    Solve (A−4I)v=0(A - 4I)\mathbf{v} = \mathbf{0}:

    [−22−21−12−11−3][v1v2v3]=[000]\begin{bmatrix} -2 & 2 & -2 \\ 1 & -1 & 2 \\ -1 & 1 & -3 \end{bmatrix} \begin{bmatrix} v_1 \\ v_2 \\ v_3 \end{bmatrix} = \begin{bmatrix} 0 \\ 0 \\ 0 \end{bmatrix}

    From the first equation, divide by -2: v1−v2+v3=0  ⟹  v1=v2−v3v_1 - v_2 + v_3 = 0 \implies v_1 = v_2 - v_3.
    Substitute v1v_1 into the second equation: (v2−v3)−v2+2v3=0  ⟹  v3=0(v_2 - v_3) - v_2 + 2v_3 = 0 \implies v_3 = 0.
    Substitute v3=0v_3 = 0 into v1=v2−v3v_1 = v_2 - v_3: v1=v2−0  ⟹  v1=v2v_1 = v_2 - 0 \implies v_1 = v_2.
    Let v2=kv_2 = k. Then v1=kv_1 = k and v3=0v_3 = 0.
    Choosing k=1k=1, the eigenvector is v3=[110]\boxed{\mathbf{v}_3 = \begin{bmatrix} 1 \\ 1 \\ 0 \end{bmatrix}}.

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