Solve {l}{2(x+y)-(x-y)=4}{4(x+y)+(x-y)=8}

$$\left\{ \begin{array} { l } { 2 ( x + y ) - ( x - y ) = 4 } \\ { 4 ( x + y ) + ( x - y ) = 8 } \end{array} \right.$$

Solve for x, y

$$x=1$$

$$y=1$$

Steps Using Substitution

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

$$2x+2y-\left(x-y\right)=4$$

To find the opposite of x-y, find the opposite of each term.

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

Combine 2x and -x to get x.

$$x+2y+y=4$$

Combine 2y and y to get 3y.

$$x+3y=4$$

Consider the second equation. Use the distributive property to multiply 4 by x+y.

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

Combine 4x and x to get 5x.

$$5x+4y-y=8$$

Combine 4y and -y to get 3y.

$$5x+3y=8$$

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.

$$x+3y=4,5x+3y=8$$

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

$$x+3y=4$$

Subtract 3y from both sides of the equation.

$$x=-3y+4$$

Substitute -3y+4 for x in the other equation, 5x+3y=8.

$$5\left(-3y+4\right)+3y=8$$

Multiply 5 times -3y+4.

$$-15y+20+3y=8$$

Add -15y to 3y.

$$-12y+20=8$$

Subtract 20 from both sides of the equation.

$$-12y=-12$$

Divide both sides by -12.

$$y=1$$

Substitute 1 for y in x=-3y+4. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=-3+4$$

Add 4 to -3.

$$x=1$$

The system is now solved.

$$x=1,y=1$$

Steps Using Matrices

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

$$2x+2y-\left(x-y\right)=4$$

To find the opposite of x-y, find the opposite of each term.

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

Combine 2x and -x to get x.

$$x+2y+y=4$$

Combine 2y and y to get 3y.

$$x+3y=4$$

Consider the second equation. Use the distributive property to multiply 4 by x+y.

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

Combine 4x and x to get 5x.

$$5x+4y-y=8$$

Combine 4y and -y to get 3y.

$$5x+3y=8$$

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

$$x+3y=4,5x+3y=8$$

Write the equations in matrix form.

$$\left(\begin{matrix}1&3\\5&3\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}4\\8\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}1&3\\5&3\end{matrix}\right).

$$inverse(\left(\begin{matrix}1&3\\5&3\end{matrix}\right))\left(\begin{matrix}1&3\\5&3\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&3\\5&3\end{matrix}\right))\left(\begin{matrix}4\\8\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&3\\5&3\end{matrix}\right))\left(\begin{matrix}4\\8\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&3\\5&3\end{matrix}\right))\left(\begin{matrix}4\\8\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{3}{3-3\times 5}&-\frac{3}{3-3\times 5}\\-\frac{5}{3-3\times 5}&\frac{1}{3-3\times 5}\end{matrix}\right)\left(\begin{matrix}4\\8\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{1}{4}&\frac{1}{4}\\\frac{5}{12}&-\frac{1}{12}\end{matrix}\right)\left(\begin{matrix}4\\8\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{1}{4}\times 4+\frac{1}{4}\times 8\\\frac{5}{12}\times 4-\frac{1}{12}\times 8\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}1\\1\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=1,y=1$$

Steps Using Elimination

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

$$2x+2y-\left(x-y\right)=4$$

To find the opposite of x-y, find the opposite of each term.

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

Combine 2x and -x to get x.

$$x+2y+y=4$$

Combine 2y and y to get 3y.

$$x+3y=4$$

Consider the second equation. Use the distributive property to multiply 4 by x+y.

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

Combine 4x and x to get 5x.

$$5x+4y-y=8$$

Combine 4y and -y to get 3y.

$$5x+3y=8$$

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.

$$x+3y=4,5x+3y=8$$

Subtract 5x+3y=8 from x+3y=4 by subtracting like terms on each side of the equal sign.

$$x-5x+3y-3y=4-8$$

Add 3y to -3y. Terms 3y and -3y cancel out, leaving an equation with only one variable that can be solved.

$$x-5x=4-8$$

Add x to -5x.

$$-4x=4-8$$

Add 4 to -8.

$$-4x=-4$$

Divide both sides by -4.

$$x=1$$

Substitute 1 for x in 5x+3y=8. Because the resulting equation contains only one variable, you can solve for y directly.

$$5+3y=8$$

Subtract 5 from both sides of the equation.

$$3y=3$$

Divide both sides by 3.

$$y=1$$

The system is now solved.

$$x=1,y=1$$