Solve {c}{2(3x-y)=2(x-5y)-64}{3(3x-2)-2y=0}

$$\left\{ \begin{array} { c } { 2 ( 3 x - y ) = 2 ( x - 5 y ) - 64 } \\ { 3 ( 3 x - 2 ) - 2 y = 0 } \end{array} \right.$$

Solve for x, y

$$x=-1$$

$$y = -\frac{15}{2} = -7\frac{1}{2} = -7.5$$

Steps Using Substitution

Consider the first equation. Use the distributive property to multiply 2 by 3x-y.

$$6x-2y=2\left(x-5y\right)-64$$

Use the distributive property to multiply 2 by x-5y.

$$6x-2y=2x-10y-64$$

Subtract 2x from both sides.

$$6x-2y-2x=-10y-64$$

Combine 6x and -2x to get 4x.

$$4x-2y=-10y-64$$

Add 10y to both sides.

$$4x-2y+10y=-64$$

Combine -2y and 10y to get 8y.

$$4x+8y=-64$$

Consider the second equation. Use the distributive property to multiply 3 by 3x-2.

$$9x-6-2y=0$$

Add 6 to both sides. Anything plus zero gives itself.

$$9x-2y=6$$

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.

$$4x+8y=-64,9x-2y=6$$

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

$$4x+8y=-64$$

Subtract 8y from both sides of the equation.

$$4x=-8y-64$$

Divide both sides by 4.

$$x=\frac{1}{4}\left(-8y-64\right)$$

Multiply \frac{1}{4} times -8y-64.

$$x=-2y-16$$

Substitute -2y-16 for x in the other equation, 9x-2y=6.

$$9\left(-2y-16\right)-2y=6$$

Multiply 9 times -2y-16.

$$-18y-144-2y=6$$

Add -18y to -2y.

$$-20y-144=6$$

Add 144 to both sides of the equation.

$$-20y=150$$

Divide both sides by -20.

$$y=-\frac{15}{2}$$

Substitute -\frac{15}{2} for y in x=-2y-16. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=-2\left(-\frac{15}{2}\right)-16$$

Multiply -2 times -\frac{15}{2}.

$$x=15-16$$

Add -16 to 15.

$$x=-1$$

The system is now solved.

$$x=-1,y=-\frac{15}{2}$$

Steps Using Matrices

Consider the first equation. Use the distributive property to multiply 2 by 3x-y.

$$6x-2y=2\left(x-5y\right)-64$$

Use the distributive property to multiply 2 by x-5y.

$$6x-2y=2x-10y-64$$

Subtract 2x from both sides.

$$6x-2y-2x=-10y-64$$

Combine 6x and -2x to get 4x.

$$4x-2y=-10y-64$$

Add 10y to both sides.

$$4x-2y+10y=-64$$

Combine -2y and 10y to get 8y.

$$4x+8y=-64$$

Consider the second equation. Use the distributive property to multiply 3 by 3x-2.

$$9x-6-2y=0$$

Add 6 to both sides. Anything plus zero gives itself.

$$9x-2y=6$$

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

$$4x+8y=-64,9x-2y=6$$

Write the equations in matrix form.

$$\left(\begin{matrix}4&8\\9&-2\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-64\\6\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}4&8\\9&-2\end{matrix}\right).

$$inverse(\left(\begin{matrix}4&8\\9&-2\end{matrix}\right))\left(\begin{matrix}4&8\\9&-2\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}4&8\\9&-2\end{matrix}\right))\left(\begin{matrix}-64\\6\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}4&8\\9&-2\end{matrix}\right))\left(\begin{matrix}-64\\6\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}4&8\\9&-2\end{matrix}\right))\left(\begin{matrix}-64\\6\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{2}{4\left(-2\right)-8\times 9}&-\frac{8}{4\left(-2\right)-8\times 9}\\-\frac{9}{4\left(-2\right)-8\times 9}&\frac{4}{4\left(-2\right)-8\times 9}\end{matrix}\right)\left(\begin{matrix}-64\\6\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{1}{40}&\frac{1}{10}\\\frac{9}{80}&-\frac{1}{20}\end{matrix}\right)\left(\begin{matrix}-64\\6\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{1}{40}\left(-64\right)+\frac{1}{10}\times 6\\\frac{9}{80}\left(-64\right)-\frac{1}{20}\times 6\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-1\\-\frac{15}{2}\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=-1,y=-\frac{15}{2}$$

Steps Using Elimination

Consider the first equation. Use the distributive property to multiply 2 by 3x-y.

$$6x-2y=2\left(x-5y\right)-64$$

Use the distributive property to multiply 2 by x-5y.

$$6x-2y=2x-10y-64$$

Subtract 2x from both sides.

$$6x-2y-2x=-10y-64$$

Combine 6x and -2x to get 4x.

$$4x-2y=-10y-64$$

Add 10y to both sides.

$$4x-2y+10y=-64$$

Combine -2y and 10y to get 8y.

$$4x+8y=-64$$

Consider the second equation. Use the distributive property to multiply 3 by 3x-2.

$$9x-6-2y=0$$

Add 6 to both sides. Anything plus zero gives itself.

$$9x-2y=6$$

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.

$$4x+8y=-64,9x-2y=6$$

To make 4x and 9x equal, multiply all terms on each side of the first equation by 9 and all terms on each side of the second by 4.

$$9\times 4x+9\times 8y=9\left(-64\right),4\times 9x+4\left(-2\right)y=4\times 6$$

Simplify.

$$36x+72y=-576,36x-8y=24$$

Subtract 36x-8y=24 from 36x+72y=-576 by subtracting like terms on each side of the equal sign.

$$36x-36x+72y+8y=-576-24$$

Add 36x to -36x. Terms 36x and -36x cancel out, leaving an equation with only one variable that can be solved.

$$72y+8y=-576-24$$

Add 72y to 8y.

$$80y=-576-24$$

Add -576 to -24.

$$80y=-600$$

Divide both sides by 80.

$$y=-\frac{15}{2}$$

Substitute -\frac{15}{2} for y in 9x-2y=6. Because the resulting equation contains only one variable, you can solve for x directly.

$$9x-2\left(-\frac{15}{2}\right)=6$$

Multiply -2 times -\frac{15}{2}.

$$9x+15=6$$

Subtract 15 from both sides of the equation.

$$9x=-9$$

Divide both sides by 9.

$$x=-1$$

The system is now solved.

$$x=-1,y=-\frac{15}{2}$$