Solve {l}{17x+23=y}{x+y=509}

$$\left\{ \begin{array} { l } { 17 x + 23 = y } \\ { x + y = 509 } \end{array} \right.$$

Solve for x, y

$$x=27$$

$$y=482$$

Steps Using Substitution

Consider the first equation. Subtract y from both sides.

$$17x+23-y=0$$

Subtract 23 from both sides. Anything subtracted from zero gives its negation.

$$17x-y=-23$$

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.

$$17x-y=-23,x+y=509$$

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

$$17x-y=-23$$

Add y to both sides of the equation.

$$17x=y-23$$

Divide both sides by 17.

$$x=\frac{1}{17}\left(y-23\right)$$

Multiply \frac{1}{17} times y-23.

$$x=\frac{1}{17}y-\frac{23}{17}$$

Substitute \frac{-23+y}{17} for x in the other equation, x+y=509.

$$\frac{1}{17}y-\frac{23}{17}+y=509$$

Add \frac{y}{17} to y.

$$\frac{18}{17}y-\frac{23}{17}=509$$

Add \frac{23}{17} to both sides of the equation.

$$\frac{18}{17}y=\frac{8676}{17}$$

Divide both sides of the equation by \frac{18}{17}, which is the same as multiplying both sides by the reciprocal of the fraction.

$$y=482$$

Substitute 482 for y in x=\frac{1}{17}y-\frac{23}{17}. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=\frac{1}{17}\times 482-\frac{23}{17}$$

Multiply \frac{1}{17} times 482.

$$x=\frac{482-23}{17}$$

Add -\frac{23}{17} to \frac{482}{17} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x=27$$

The system is now solved.

$$x=27,y=482$$

Steps Using Matrices

Consider the first equation. Subtract y from both sides.

$$17x+23-y=0$$

Subtract 23 from both sides. Anything subtracted from zero gives its negation.

$$17x-y=-23$$

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

$$17x-y=-23,x+y=509$$

Write the equations in matrix form.

$$\left(\begin{matrix}17&-1\\1&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-23\\509\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}17&-1\\1&1\end{matrix}\right).

$$inverse(\left(\begin{matrix}17&-1\\1&1\end{matrix}\right))\left(\begin{matrix}17&-1\\1&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}17&-1\\1&1\end{matrix}\right))\left(\begin{matrix}-23\\509\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}17&-1\\1&1\end{matrix}\right))\left(\begin{matrix}-23\\509\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}17&-1\\1&1\end{matrix}\right))\left(\begin{matrix}-23\\509\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{1}{17-\left(-1\right)}&-\frac{-1}{17-\left(-1\right)}\\-\frac{1}{17-\left(-1\right)}&\frac{17}{17-\left(-1\right)}\end{matrix}\right)\left(\begin{matrix}-23\\509\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{1}{18}&\frac{1}{18}\\-\frac{1}{18}&\frac{17}{18}\end{matrix}\right)\left(\begin{matrix}-23\\509\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{1}{18}\left(-23\right)+\frac{1}{18}\times 509\\-\frac{1}{18}\left(-23\right)+\frac{17}{18}\times 509\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}27\\482\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=27,y=482$$

Steps Using Elimination

Consider the first equation. Subtract y from both sides.

$$17x+23-y=0$$

Subtract 23 from both sides. Anything subtracted from zero gives its negation.

$$17x-y=-23$$

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.

$$17x-y=-23,x+y=509$$

To make 17x and x equal, multiply all terms on each side of the first equation by 1 and all terms on each side of the second by 17.

$$17x-y=-23,17x+17y=17\times 509$$

Simplify.

$$17x-y=-23,17x+17y=8653$$

Subtract 17x+17y=8653 from 17x-y=-23 by subtracting like terms on each side of the equal sign.

$$17x-17x-y-17y=-23-8653$$

Add 17x to -17x. Terms 17x and -17x cancel out, leaving an equation with only one variable that can be solved.

$$-y-17y=-23-8653$$

Add -y to -17y.

$$-18y=-23-8653$$

Add -23 to -8653.

$$-18y=-8676$$

Divide both sides by -18.

$$y=482$$

Substitute 482 for y in x+y=509. Because the resulting equation contains only one variable, you can solve for x directly.

$$x+482=509$$

Subtract 482 from both sides of the equation.

$$x=27$$

The system is now solved.

$$x=27,y=482$$