Solve {l}{4(2x-y)-7(2y+x)=-36}{-2(x+2)-7y=-18}

$$\left\{ \begin{array} { l } { 4 ( 2 x - y ) - 7 ( 2 y + x ) = - 36 } \\ { - 2 ( x + 2 ) - 7 y = - 18 } \end{array} \right.$$

Solve for x, y

$$x=0$$

$$y=2$$

Steps Using Substitution

Consider the first equation. Use the distributive property to multiply 4 by 2x-y.

$$8x-4y-7\left(2y+x\right)=-36$$

Use the distributive property to multiply -7 by 2y+x.

$$8x-4y-14y-7x=-36$$

Combine -4y and -14y to get -18y.

$$8x-18y-7x=-36$$

Combine 8x and -7x to get x.

$$x-18y=-36$$

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

$$-2x-4-7y=-18$$

Add 4 to both sides.

$$-2x-7y=-18+4$$

Add -18 and 4 to get -14.

$$-2x-7y=-14$$

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.

$$x-18y=-36,-2x-7y=-14$$

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

$$x-18y=-36$$

Add 18y to both sides of the equation.

$$x=18y-36$$

Substitute -36+18y for x in the other equation, -2x-7y=-14.

$$-2\left(18y-36\right)-7y=-14$$

Multiply -2 times -36+18y.

$$-36y+72-7y=-14$$

Add -36y to -7y.

$$-43y+72=-14$$

Subtract 72 from both sides of the equation.

$$-43y=-86$$

Divide both sides by -43.

$$y=2$$

Substitute 2 for y in x=18y-36. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=18\times 2-36$$

Multiply 18 times 2.

$$x=36-36$$

Add -36 to 36.

$$x=0$$

The system is now solved.

$$x=0,y=2$$

Steps Using Matrices

Consider the first equation. Use the distributive property to multiply 4 by 2x-y.

$$8x-4y-7\left(2y+x\right)=-36$$

Use the distributive property to multiply -7 by 2y+x.

$$8x-4y-14y-7x=-36$$

Combine -4y and -14y to get -18y.

$$8x-18y-7x=-36$$

Combine 8x and -7x to get x.

$$x-18y=-36$$

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

$$-2x-4-7y=-18$$

Add 4 to both sides.

$$-2x-7y=-18+4$$

Add -18 and 4 to get -14.

$$-2x-7y=-14$$

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

$$x-18y=-36,-2x-7y=-14$$

Write the equations in matrix form.

$$\left(\begin{matrix}1&-18\\-2&-7\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-36\\-14\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}1&-18\\-2&-7\end{matrix}\right).

$$inverse(\left(\begin{matrix}1&-18\\-2&-7\end{matrix}\right))\left(\begin{matrix}1&-18\\-2&-7\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&-18\\-2&-7\end{matrix}\right))\left(\begin{matrix}-36\\-14\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&-18\\-2&-7\end{matrix}\right))\left(\begin{matrix}-36\\-14\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&-18\\-2&-7\end{matrix}\right))\left(\begin{matrix}-36\\-14\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{7}{-7-\left(-18\left(-2\right)\right)}&-\frac{-18}{-7-\left(-18\left(-2\right)\right)}\\-\frac{-2}{-7-\left(-18\left(-2\right)\right)}&\frac{1}{-7-\left(-18\left(-2\right)\right)}\end{matrix}\right)\left(\begin{matrix}-36\\-14\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{7}{43}&-\frac{18}{43}\\-\frac{2}{43}&-\frac{1}{43}\end{matrix}\right)\left(\begin{matrix}-36\\-14\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{7}{43}\left(-36\right)-\frac{18}{43}\left(-14\right)\\-\frac{2}{43}\left(-36\right)-\frac{1}{43}\left(-14\right)\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}0\\2\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=0,y=2$$

Steps Using Elimination

Consider the first equation. Use the distributive property to multiply 4 by 2x-y.

$$8x-4y-7\left(2y+x\right)=-36$$

Use the distributive property to multiply -7 by 2y+x.

$$8x-4y-14y-7x=-36$$

Combine -4y and -14y to get -18y.

$$8x-18y-7x=-36$$

Combine 8x and -7x to get x.

$$x-18y=-36$$

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

$$-2x-4-7y=-18$$

Add 4 to both sides.

$$-2x-7y=-18+4$$

Add -18 and 4 to get -14.

$$-2x-7y=-14$$

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.

$$x-18y=-36,-2x-7y=-14$$

To make x and -2x equal, multiply all terms on each side of the first equation by -2 and all terms on each side of the second by 1.

$$-2x-2\left(-18\right)y=-2\left(-36\right),-2x-7y=-14$$

Simplify.

$$-2x+36y=72,-2x-7y=-14$$

Subtract -2x-7y=-14 from -2x+36y=72 by subtracting like terms on each side of the equal sign.

$$-2x+2x+36y+7y=72+14$$

Add -2x to 2x. Terms -2x and 2x cancel out, leaving an equation with only one variable that can be solved.

$$36y+7y=72+14$$

Add 36y to 7y.

$$43y=72+14$$

Add 72 to 14.

$$43y=86$$

Divide both sides by 43.

$$y=2$$

Substitute 2 for y in -2x-7y=-14. Because the resulting equation contains only one variable, you can solve for x directly.

$$-2x-7\times 2=-14$$

Multiply -7 times 2.

$$-2x-14=-14$$

Add 14 to both sides of the equation.

$$-2x=0$$

Divide both sides by -2.

$$x=0$$

The system is now solved.

$$x=0,y=2$$