X+Y=7.2; -X+Y=4.8

asked by guest
on Jan 13, 2025 at 11:01 pm



You asked:

Solve the system of equations:X+Y=7.2X+Y=4.8\begin{aligned}X + Y &= 7.2\\-X + Y &= 4.8\end{aligned}

MathBot Answer:

The system of equations has one solution.X=65X = \frac{6}{5} Y=6Y = 6


Solve by substitution:

Solve X+Y=365X + Y = \frac{36}{5} for XX. X=365YX = \frac{36}{5} - Y

Substitute 365Y\frac{36}{5} - Y for XX in X+Y=245- X + Y = \frac{24}{5} and simplify. X+Y=245(365Y)+Y=245Y=6\begin{aligned}- X + Y &= \frac{24}{5} \\ - \left(\frac{36}{5} - Y\right) + Y &= \frac{24}{5} \\ Y &= 6 \end{aligned}

Use substitution of the numerical value of YY to get the values of XX. X=365YX=(1)6+365X=65\begin{aligned}X &= \frac{36}{5} - Y \\ X &= \left(-1\right) 6 + \frac{36}{5} \\ X &= \frac{6}{5}\end{aligned}


Solve by Gauss-Jordan Elimination:

Begin by writing the augmented matrix of the system of equations. [1136511245]\begin{bmatrix}1 & 1 &\bigm |& \frac{36}{5}\\-1 & 1 &\bigm |& \frac{24}{5}\end{bmatrix}

Then use a series of elementary row operations to convert the matrix into reduced-row echelon form. The three elementary row operations are:

  1. Swap the positions of any two rows.

  2. Multiply any row by a nonzero scalar.

  3. Multiply a row by a nonzero scalar and add it to any other row.


First, switch the rows in the matrix such that the row with the leftmost non-zero entry with the greatest magnitude is at the top.

[1136511245]\begin{bmatrix}1 & 1 &\bigm |& \frac{36}{5}\\-1 & 1 &\bigm |& \frac{24}{5}\end{bmatrix}

The leading term of row 11 is already 11 so this row does not need to be multiplied by a scalar.

[1136511245]\begin{bmatrix}1 & 1 &\bigm |& \frac{36}{5}\\-1 & 1 &\bigm |& \frac{24}{5}\end{bmatrix}

Multiply row 11 by scalar 11 and add it to row 22.

[113650212]\begin{bmatrix}1 & 1 &\bigm |& \frac{36}{5}\\0 & 2 &\bigm |& 12\end{bmatrix}

Multiply row 22 by scalar 12\frac{1}{2} to make the leading term 11.

[11365016]\begin{bmatrix}1 & 1 &\bigm |& \frac{36}{5}\\0 & 1 &\bigm |& 6\end{bmatrix}

Multiply row 22 by scalar 1-1 and add it to row 11.

[1065016]\begin{bmatrix}1 & 0 &\bigm |& \frac{6}{5}\\0 & 1 &\bigm |& 6\end{bmatrix}

Once the matrix is in reduced-row echelon form, convert the matrix back into linear equations to find the solution. 1X+0Y=65X=65\begin{aligned}1 \cdot X+ 0 \cdot Y = \frac{6}{5} \\ X = \frac{6}{5}\end{aligned}0X+1Y=6Y=6\begin{aligned}0 \cdot X+ 1 \cdot Y = 6 \\ Y = 6\end{aligned}


Solve by matrix inversion:

In cases where the coefficient matrix of the system of equations is invertible, we can use the inverse to solve the system. Use this method with care as matrix inversion can be numerically unstable for ill-conditioned matrices.


Express the linear equations in the form A×X=BA \times X = B where AA is the coefficient matrix, XX is the matrix of unknowns, and BB is the constant matrix.[1111]×[XY]=[365245]\left[\begin{matrix}1 & 1\\-1 & 1\end{matrix}\right] \times \left[\begin{matrix}X\\Y\end{matrix}\right] = \left[\begin{matrix}\frac{36}{5}\\\frac{24}{5}\end{matrix}\right]

The product of AA and its inverse A1A^{-1} is the identity matrix. Any matrix multiplied by the identity matrix remains unchanged, so this yields the matrix of unknowns on the left hand side of the equation, and the solution matrix on the right. A×X=BA1×A×X=A1×BI×X=A1×BX=A1×B\begin{aligned} A \times X &= B\\ A^{-1} \times A \times X &= A^{-1} \times B \\ I \times X &= A^{-1} \times B \\ X &= A^{-1} \times B \end{aligned}

Using a computer algebra system, calculate A1A^{-1}. [12121212]\left[\begin{matrix}\frac{1}{2} & - \frac{1}{2}\\\frac{1}{2} & \frac{1}{2}\end{matrix}\right]

Multiply both sides of the equation by the inverse. [12121212]×[1111]×[XY]=[12121212]×[365245]\left[\begin{matrix}\frac{1}{2} & - \frac{1}{2}\\\frac{1}{2} & \frac{1}{2}\end{matrix}\right] \times \left[\begin{matrix}1 & 1\\-1 & 1\end{matrix}\right] \times \left[\begin{matrix}X\\Y\end{matrix}\right] = \left[\begin{matrix}\frac{1}{2} & - \frac{1}{2}\\\frac{1}{2} & \frac{1}{2}\end{matrix}\right] \times \left[\begin{matrix}\frac{36}{5}\\\frac{24}{5}\end{matrix}\right] [XY]=[656]\left[\begin{matrix}X\\Y\end{matrix}\right] = \left[\begin{matrix}\frac{6}{5}\\6\end{matrix}\right]

X=65X = \frac{6}{5}Y=6Y = 6

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