Solve {l}{18x-16y=-312}{78x-16y=408}

$$\left. \begin{array} { l } { 18 x - 16 y = - 312 } \\ { 78 x - 16 y = 408 } \end{array} \right.$$

Solve for x, y

$$x=12$$

$$y=33$$

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.

$$18x-16y=-312,78x-16y=408$$

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

$$18x-16y=-312$$

Add 16y to both sides of the equation.

$$18x=16y-312$$

Divide both sides by 18.

$$x=\frac{1}{18}\left(16y-312\right)$$

Multiply \frac{1}{18} times 16y-312.

$$x=\frac{8}{9}y-\frac{52}{3}$$

Substitute \frac{8y}{9}-\frac{52}{3} for x in the other equation, 78x-16y=408.

$$78\left(\frac{8}{9}y-\frac{52}{3}\right)-16y=408$$

Multiply 78 times \frac{8y}{9}-\frac{52}{3}.

$$\frac{208}{3}y-1352-16y=408$$

Add \frac{208y}{3} to -16y.

$$\frac{160}{3}y-1352=408$$

Add 1352 to both sides of the equation.

$$\frac{160}{3}y=1760$$

Divide both sides of the equation by \frac{160}{3}, which is the same as multiplying both sides by the reciprocal of the fraction.

$$y=33$$

Substitute 33 for y in x=\frac{8}{9}y-\frac{52}{3}. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=\frac{8}{9}\times 33-\frac{52}{3}$$

Multiply \frac{8}{9} times 33.

$$x=\frac{88-52}{3}$$

Add -\frac{52}{3} to \frac{88}{3} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x=12$$

The system is now solved.

$$x=12,y=33$$

Steps Using Matrices

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

$$18x-16y=-312,78x-16y=408$$

Write the equations in matrix form.

$$\left(\begin{matrix}18&-16\\78&-16\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-312\\408\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}18&-16\\78&-16\end{matrix}\right).

$$inverse(\left(\begin{matrix}18&-16\\78&-16\end{matrix}\right))\left(\begin{matrix}18&-16\\78&-16\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}18&-16\\78&-16\end{matrix}\right))\left(\begin{matrix}-312\\408\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}18&-16\\78&-16\end{matrix}\right))\left(\begin{matrix}-312\\408\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}18&-16\\78&-16\end{matrix}\right))\left(\begin{matrix}-312\\408\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{16}{18\left(-16\right)-\left(-16\times 78\right)}&-\frac{-16}{18\left(-16\right)-\left(-16\times 78\right)}\\-\frac{78}{18\left(-16\right)-\left(-16\times 78\right)}&\frac{18}{18\left(-16\right)-\left(-16\times 78\right)}\end{matrix}\right)\left(\begin{matrix}-312\\408\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{1}{60}&\frac{1}{60}\\-\frac{13}{160}&\frac{3}{160}\end{matrix}\right)\left(\begin{matrix}-312\\408\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{1}{60}\left(-312\right)+\frac{1}{60}\times 408\\-\frac{13}{160}\left(-312\right)+\frac{3}{160}\times 408\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}12\\33\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=12,y=33$$

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.

$$18x-16y=-312,78x-16y=408$$

Subtract 78x-16y=408 from 18x-16y=-312 by subtracting like terms on each side of the equal sign.

$$18x-78x-16y+16y=-312-408$$

Add -16y to 16y. Terms -16y and 16y cancel out, leaving an equation with only one variable that can be solved.

$$18x-78x=-312-408$$

Add 18x to -78x.

$$-60x=-312-408$$

Add -312 to -408.

$$-60x=-720$$

Divide both sides by -60.

$$x=12$$

Substitute 12 for x in 78x-16y=408. Because the resulting equation contains only one variable, you can solve for y directly.

$$78\times 12-16y=408$$

Multiply 78 times 12.

$$936-16y=408$$

Subtract 936 from both sides of the equation.

$$-16y=-528$$

Divide both sides by -16.

$$y=33$$

The system is now solved.

$$x=12,y=33$$