Answer to Question #233584 in Linear Algebra for rabiatul

Question #233584

A curve


y  ax  bx  c

2


where a, b and c are constants, passes


through the points (2,11), (-1,-16) and (3,28).

(a) By using the above information, construct a system

containing three linear equations.

(b) Express the above system as a matrix equation


AX  B.

(c) Find the inverse of matrix A by using the adjoint matrix

method. Hence, obtain the values of a, b and c.


1
Expert's answer
2021-09-06T14:29:01-0400
y=ax2+bx+cy=ax^2+bx+c

(a)


a(2)2+b(2)+c=11a(2)^2+b(2)+c=11

a(1)2+b(1)+c=16a(-1)^2+b(-1)+c=-16




a(3)2+b(3)+c=28a(3)^2+b(3)+c=28

Construct a system containing three linear equations.


4a+2b+c=114a+2b+c=11ab+c=16a-b+c=-169a+3b+c=289a+3b+c=28

(b)


AX=BAX=B


A=(421111931),X=(abc),B=(111628)A=\begin{pmatrix} 4 & 2 & 1 \\ 1 & -1 & 1 \\ 9 & 3 & 1 \\ \end{pmatrix}, X=\begin{pmatrix} a \\ b \\ c \end{pmatrix}, B=\begin{pmatrix} 11\\ -16 \\ 28 \end{pmatrix}

(421111931)(abc)=(111628)\begin{pmatrix} 4 & 2 & 1 \\ 1 & -1 & 1 \\ 9 & 3 & 1 \\ \end{pmatrix}\begin{pmatrix} a \\ b \\ c \end{pmatrix}=\begin{pmatrix} 11\\ -16 \\ 28 \end{pmatrix}

(c)


AX=BAX=B

A1AX=A1BA^{-1}AX=A^{-1}B

X=A1BX=A^{-1}B

detA=421111931=4113121191+11193\det A=\begin{vmatrix} 4 & 2 & 1 \\ 1 & -1 & 1 \\ 9 & 3 & 1 \\ \end{vmatrix}=4\begin{vmatrix} -1 & 1 \\ 3 & 1 \end{vmatrix}-2\begin{vmatrix} 1 & 1 \\ 9 & 1 \end{vmatrix}+1\begin{vmatrix} 1 & -1 \\ 9 & 3 \end{vmatrix}

=4(13)2(19)+(3+9)=4(-1-3)-2(1-9)+(3+9)

=16+16+12=120=>A1 exists=-16+16+12=12\not=0=>A^{-1}\ exists


Find the cofactor matrix:


C11=(1)1+11131=4C_{11}=(-1)^{1+1}\begin{vmatrix} -1 & 1 \\ 3 & 1 \end{vmatrix}=-4

C12=(1)1+21191=8C_{12}=(-1)^{1+2}\begin{vmatrix} 1 & 1 \\ 9 & 1 \end{vmatrix}=8

C13=(1)1+31193=12C_{13}=(-1)^{1+3}\begin{vmatrix} 1 & -1 \\ 9 & 3 \end{vmatrix}=12

C21=(1)2+12131=1C_{21}=(-1)^{2+1}\begin{vmatrix} 2 & 1 \\ 3 & 1 \end{vmatrix}=1

C22(1)2+24191=5C_{22}(-1)^{2+2}\begin{vmatrix} 4 & 1 \\ 9 & 1 \end{vmatrix}=-5

C23=(1)2+34293=6C_{23}=(-1)^{2+3}\begin{vmatrix} 4 & 2 \\ 9 & 3 \end{vmatrix}=6

C31=(1)3+12111=3C_{31}=(-1)^{3+1}\begin{vmatrix} 2 & 1 \\ -1 & 1 \end{vmatrix}=3

C32=(1)3+24111=3C_{32}=(-1)^{3+2}\begin{vmatrix} 4 & 1 \\ 1 & 1 \end{vmatrix}=-3

C33=(1)3+34211=6C_{33}=(-1)^{3+3}\begin{vmatrix} 4 & 2 \\ 1 & -1 \end{vmatrix}=-6

The cofactor matrix is


C=(4812156336)C=\begin{pmatrix} -4 &8 & 12 \\ 1 & -5 & 6 \\ 3 & -3 & -6 \\ \end{pmatrix}

The transpose of the cofactor matrix is


CT=(4138531266)C^T=\begin{pmatrix} -4 & 1 & 3\\ 8 & -5 & -3 \\ 12 & 6 & -6 \\ \end{pmatrix}

A1=112(4138531266)A^{-1}=\dfrac{1}{12}\begin{pmatrix} -4 & 1 & 3\\ 8 & -5 & -3 \\ 12 & 6 & -6 \\ \end{pmatrix}

A1=(1/31/121/42/35/121/411/21/2)A^{-1}=\begin{pmatrix} -1/3 & 1/12 & 1/4\\ 2/3 & -5/12 & -1/4 \\ 1 & 1 /2& -1/2 \\ \end{pmatrix}

(abc)=(1/31/121/42/35/121/411/21/2)(111628)\begin{pmatrix} a \\ b \\ c \end{pmatrix}=\begin{pmatrix} -1/3 & 1/12 & 1/4\\ 2/3 & -5/12 & -1/4 \\ 1 & 1 /2& -1/2 \\ \end{pmatrix}\begin{pmatrix} 11\\ -16 \\ 28 \end{pmatrix}

=(11/34/3+722/3+20/3711814)=(2711)=\begin{pmatrix} -11/3-4/3+7\\ 22/3+20/3-7 \\ 11-8-14 \end{pmatrix}=\begin{pmatrix} 2\\ 7 \\ -11 \end{pmatrix}

a=2,b=7,c=11a=2, b=7, c=-11

y=2x2+7x11y=2x^2+7x-11


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