Solve {r}{2(x+2y)+3(3x-y)=38}{4(3x+2y)-3(x+5y)=-8}

$$\left. \begin{array} { r } { 2 ( x + 2 y ) + 3 ( 3 x - y ) = 38 } \\ { 4 ( 3 x + 2 y ) - 3 ( x + 5 y ) = - 8 } \end{array} \right.$$

Solve for x, y

$$x=3$$

$$y=5$$

Steps Using Substitution

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

$$2x+4y+3\left(3x-y\right)=38$$

Use the distributive property to multiply 3 by 3x-y.

$$2x+4y+9x-3y=38$$

Combine 2x and 9x to get 11x.

$$11x+4y-3y=38$$

Combine 4y and -3y to get y.

$$11x+y=38$$

Consider the second equation. Use the distributive property to multiply 4 by 3x+2y.

$$12x+8y-3\left(x+5y\right)=-8$$

Use the distributive property to multiply -3 by x+5y.

$$12x+8y-3x-15y=-8$$

Combine 12x and -3x to get 9x.

$$9x+8y-15y=-8$$

Combine 8y and -15y to get -7y.

$$9x-7y=-8$$

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.

$$11x+y=38,9x-7y=-8$$

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

$$11x+y=38$$

Subtract y from both sides of the equation.

$$11x=-y+38$$

Divide both sides by 11.

$$x=\frac{1}{11}\left(-y+38\right)$$

Multiply \frac{1}{11} times -y+38.

$$x=-\frac{1}{11}y+\frac{38}{11}$$

Substitute \frac{-y+38}{11} for x in the other equation, 9x-7y=-8.

$$9\left(-\frac{1}{11}y+\frac{38}{11}\right)-7y=-8$$

Multiply 9 times \frac{-y+38}{11}.

$$-\frac{9}{11}y+\frac{342}{11}-7y=-8$$

Add -\frac{9y}{11} to -7y.

$$-\frac{86}{11}y+\frac{342}{11}=-8$$

Subtract \frac{342}{11} from both sides of the equation.

$$-\frac{86}{11}y=-\frac{430}{11}$$

Divide both sides of the equation by -\frac{86}{11}, which is the same as multiplying both sides by the reciprocal of the fraction.

$$y=5$$

Substitute 5 for y in x=-\frac{1}{11}y+\frac{38}{11}. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=-\frac{1}{11}\times 5+\frac{38}{11}$$

Multiply -\frac{1}{11} times 5.

$$x=\frac{-5+38}{11}$$

Add \frac{38}{11} to -\frac{5}{11} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x=3$$

The system is now solved.

$$x=3,y=5$$

Steps Using Matrices

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

$$2x+4y+3\left(3x-y\right)=38$$

Use the distributive property to multiply 3 by 3x-y.

$$2x+4y+9x-3y=38$$

Combine 2x and 9x to get 11x.

$$11x+4y-3y=38$$

Combine 4y and -3y to get y.

$$11x+y=38$$

Consider the second equation. Use the distributive property to multiply 4 by 3x+2y.

$$12x+8y-3\left(x+5y\right)=-8$$

Use the distributive property to multiply -3 by x+5y.

$$12x+8y-3x-15y=-8$$

Combine 12x and -3x to get 9x.

$$9x+8y-15y=-8$$

Combine 8y and -15y to get -7y.

$$9x-7y=-8$$

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

$$11x+y=38,9x-7y=-8$$

Write the equations in matrix form.

$$\left(\begin{matrix}11&1\\9&-7\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}38\\-8\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}11&1\\9&-7\end{matrix}\right).

$$inverse(\left(\begin{matrix}11&1\\9&-7\end{matrix}\right))\left(\begin{matrix}11&1\\9&-7\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}11&1\\9&-7\end{matrix}\right))\left(\begin{matrix}38\\-8\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}11&1\\9&-7\end{matrix}\right))\left(\begin{matrix}38\\-8\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}11&1\\9&-7\end{matrix}\right))\left(\begin{matrix}38\\-8\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{7}{11\left(-7\right)-9}&-\frac{1}{11\left(-7\right)-9}\\-\frac{9}{11\left(-7\right)-9}&\frac{11}{11\left(-7\right)-9}\end{matrix}\right)\left(\begin{matrix}38\\-8\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{7}{86}&\frac{1}{86}\\\frac{9}{86}&-\frac{11}{86}\end{matrix}\right)\left(\begin{matrix}38\\-8\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{7}{86}\times 38+\frac{1}{86}\left(-8\right)\\\frac{9}{86}\times 38-\frac{11}{86}\left(-8\right)\end{matrix}\right)$$

Do the arithmetic.

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

Extract the matrix elements x and y.

$$x=3,y=5$$

Steps Using Elimination

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

$$2x+4y+3\left(3x-y\right)=38$$

Use the distributive property to multiply 3 by 3x-y.

$$2x+4y+9x-3y=38$$

Combine 2x and 9x to get 11x.

$$11x+4y-3y=38$$

Combine 4y and -3y to get y.

$$11x+y=38$$

Consider the second equation. Use the distributive property to multiply 4 by 3x+2y.

$$12x+8y-3\left(x+5y\right)=-8$$

Use the distributive property to multiply -3 by x+5y.

$$12x+8y-3x-15y=-8$$

Combine 12x and -3x to get 9x.

$$9x+8y-15y=-8$$

Combine 8y and -15y to get -7y.

$$9x-7y=-8$$

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.

$$11x+y=38,9x-7y=-8$$

To make 11x 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 11.

$$9\times 11x+9y=9\times 38,11\times 9x+11\left(-7\right)y=11\left(-8\right)$$

Simplify.

$$99x+9y=342,99x-77y=-88$$

Subtract 99x-77y=-88 from 99x+9y=342 by subtracting like terms on each side of the equal sign.

$$99x-99x+9y+77y=342+88$$

Add 99x to -99x. Terms 99x and -99x cancel out, leaving an equation with only one variable that can be solved.

$$9y+77y=342+88$$

Add 9y to 77y.

$$86y=342+88$$

Add 342 to 88.

$$86y=430$$

Divide both sides by 86.

$$y=5$$

Substitute 5 for y in 9x-7y=-8. Because the resulting equation contains only one variable, you can solve for x directly.

$$9x-7\times 5=-8$$

Multiply -7 times 5.

$$9x-35=-8$$

Add 35 to both sides of the equation.

$$9x=27$$

Divide both sides by 9.

$$x=3$$

The system is now solved.

$$x=3,y=5$$