Solve {l}{5x-y=1200}{2y-11x=120(-3)}

$$\left\{ \begin{array} { l } { 5 x - y = 1200 } \\ { 2 y - 11 x = 120 ( - 3 ) } \end{array} \right.$$

Solve for x, y

$$x=-2040$$

$$y=-11400$$

Steps Using Substitution

To solve a pair of equations using substitution, first solve one of the equations for one of the variables. Then substitute the result for that variable in the other equation.

$$5x-y=1200,-11x+2y=-360$$

Choose one of the equations and solve it for x by isolating x on the left hand side of the equal sign.

$$5x-y=1200$$

Add y to both sides of the equation.

$$5x=y+1200$$

Divide both sides by 5.

$$x=\frac{1}{5}\left(y+1200\right)$$

Multiply \frac{1}{5} times y+1200.

$$x=\frac{1}{5}y+240$$

Substitute \frac{y}{5}+240 for x in the other equation, -11x+2y=-360.

$$-11\left(\frac{1}{5}y+240\right)+2y=-360$$

Multiply -11 times \frac{y}{5}+240.

$$-\frac{11}{5}y-2640+2y=-360$$

Add -\frac{11y}{5} to 2y.

$$-\frac{1}{5}y-2640=-360$$

Add 2640 to both sides of the equation.

$$-\frac{1}{5}y=2280$$

Multiply both sides by -5.

$$y=-11400$$

Substitute -11400 for y in x=\frac{1}{5}y+240. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=\frac{1}{5}\left(-11400\right)+240$$

Multiply \frac{1}{5} times -11400.

$$x=-2280+240$$

Add 240 to -2280.

$$x=-2040$$

The system is now solved.

$$x=-2040,y=-11400$$

Steps Using Matrices

Put the equations in standard form and then use matrices to solve the system of equations.

$$5x-y=1200,-11x+2y=-360$$

Write the equations in matrix form.

$$\left(\begin{matrix}5&-1\\-11&2\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}1200\\-360\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}5&-1\\-11&2\end{matrix}\right).

$$inverse(\left(\begin{matrix}5&-1\\-11&2\end{matrix}\right))\left(\begin{matrix}5&-1\\-11&2\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}5&-1\\-11&2\end{matrix}\right))\left(\begin{matrix}1200\\-360\end{matrix}\right)$$

The product of a matrix and its inverse is the identity matrix.

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}5&-1\\-11&2\end{matrix}\right))\left(\begin{matrix}1200\\-360\end{matrix}\right)$$

Multiply the matrices on the left hand side of the equal sign.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}5&-1\\-11&2\end{matrix}\right))\left(\begin{matrix}1200\\-360\end{matrix}\right)$$

For the 2\times 2 matrix \left(\begin{matrix}a&b\\c&d\end{matrix}\right), the inverse matrix is \left(\begin{matrix}\frac{d}{ad-bc}&\frac{-b}{ad-bc}\\\frac{-c}{ad-bc}&\frac{a}{ad-bc}\end{matrix}\right), so the matrix equation can be rewritten as a matrix multiplication problem.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{2}{5\times 2-\left(-\left(-11\right)\right)}&-\frac{-1}{5\times 2-\left(-\left(-11\right)\right)}\\-\frac{-11}{5\times 2-\left(-\left(-11\right)\right)}&\frac{5}{5\times 2-\left(-\left(-11\right)\right)}\end{matrix}\right)\left(\begin{matrix}1200\\-360\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-2&-1\\-11&-5\end{matrix}\right)\left(\begin{matrix}1200\\-360\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-2\times 1200-\left(-360\right)\\-11\times 1200-5\left(-360\right)\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-2040\\-11400\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=-2040,y=-11400$$

Steps Using Elimination

In order to solve by elimination, coefficients of one of the variables must be the same in both equations so that the variable will cancel out when one equation is subtracted from the other.

$$5x-y=1200,-11x+2y=-360$$

To make 5x and -11x equal, multiply all terms on each side of the first equation by -11 and all terms on each side of the second by 5.

$$-11\times 5x-11\left(-1\right)y=-11\times 1200,5\left(-11\right)x+5\times 2y=5\left(-360\right)$$

Simplify.

$$-55x+11y=-13200,-55x+10y=-1800$$

Subtract -55x+10y=-1800 from -55x+11y=-13200 by subtracting like terms on each side of the equal sign.

$$-55x+55x+11y-10y=-13200+1800$$

Add -55x to 55x. Terms -55x and 55x cancel out, leaving an equation with only one variable that can be solved.

$$11y-10y=-13200+1800$$

Add 11y to -10y.

$$y=-13200+1800$$

Add -13200 to 1800.

$$y=-11400$$

Substitute -11400 for y in -11x+2y=-360. Because the resulting equation contains only one variable, you can solve for x directly.

$$-11x+2\left(-11400\right)=-360$$

Multiply 2 times -11400.

$$-11x-22800=-360$$

Add 22800 to both sides of the equation.

$$-11x=22440$$

Divide both sides by -11.

$$x=-2040$$

The system is now solved.

$$x=-2040,y=-11400$$