Solve {l}{987x+123y=741}{321x-123y=667}

$$\left\{ \begin{array} { l } { 987 x + 123 y = 741 } \\ { 321 x - 123 y = 667 } \end{array} \right.$$

Solve for x, y

$$x = \frac{352}{327} = 1\frac{25}{327} \approx 1.076452599$$

$$y = -\frac{35039}{13407} = -2\frac{8225}{13407} \approx -2.613485493$$

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.

$$987x+123y=741,321x-123y=667$$

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

$$987x+123y=741$$

Subtract 123y from both sides of the equation.

$$987x=-123y+741$$

Divide both sides by 987.

$$x=\frac{1}{987}\left(-123y+741\right)$$

Multiply \frac{1}{987} times -123y+741.

$$x=-\frac{41}{329}y+\frac{247}{329}$$

Substitute \frac{-41y+247}{329} for x in the other equation, 321x-123y=667.

$$321\left(-\frac{41}{329}y+\frac{247}{329}\right)-123y=667$$

Multiply 321 times \frac{-41y+247}{329}.

$$-\frac{13161}{329}y+\frac{79287}{329}-123y=667$$

Add -\frac{13161y}{329} to -123y.

$$-\frac{53628}{329}y+\frac{79287}{329}=667$$

Subtract \frac{79287}{329} from both sides of the equation.

$$-\frac{53628}{329}y=\frac{140156}{329}$$

Divide both sides of the equation by -\frac{53628}{329}, which is the same as multiplying both sides by the reciprocal of the fraction.

$$y=-\frac{35039}{13407}$$

Substitute -\frac{35039}{13407} for y in x=-\frac{41}{329}y+\frac{247}{329}. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=-\frac{41}{329}\left(-\frac{35039}{13407}\right)+\frac{247}{329}$$

Multiply -\frac{41}{329} times -\frac{35039}{13407} by multiplying numerator times numerator and denominator times denominator. Then reduce the fraction to lowest terms if possible.

$$x=\frac{35039}{107583}+\frac{247}{329}$$

Add \frac{247}{329} to \frac{35039}{107583} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x=\frac{352}{327}$$

The system is now solved.

$$x=\frac{352}{327},y=-\frac{35039}{13407}$$

Steps Using Matrices

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

$$987x+123y=741,321x-123y=667$$

Write the equations in matrix form.

$$\left(\begin{matrix}987&123\\321&-123\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}741\\667\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}987&123\\321&-123\end{matrix}\right).

$$inverse(\left(\begin{matrix}987&123\\321&-123\end{matrix}\right))\left(\begin{matrix}987&123\\321&-123\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}987&123\\321&-123\end{matrix}\right))\left(\begin{matrix}741\\667\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}987&123\\321&-123\end{matrix}\right))\left(\begin{matrix}741\\667\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}987&123\\321&-123\end{matrix}\right))\left(\begin{matrix}741\\667\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{123}{987\left(-123\right)-123\times 321}&-\frac{123}{987\left(-123\right)-123\times 321}\\-\frac{321}{987\left(-123\right)-123\times 321}&\frac{987}{987\left(-123\right)-123\times 321}\end{matrix}\right)\left(\begin{matrix}741\\667\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{1}{1308}&\frac{1}{1308}\\\frac{107}{53628}&-\frac{329}{53628}\end{matrix}\right)\left(\begin{matrix}741\\667\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{1}{1308}\times 741+\frac{1}{1308}\times 667\\\frac{107}{53628}\times 741-\frac{329}{53628}\times 667\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{352}{327}\\-\frac{35039}{13407}\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=\frac{352}{327},y=-\frac{35039}{13407}$$

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.

$$987x+123y=741,321x-123y=667$$

To make 987x and 321x equal, multiply all terms on each side of the first equation by 321 and all terms on each side of the second by 987.

$$321\times 987x+321\times 123y=321\times 741,987\times 321x+987\left(-123\right)y=987\times 667$$

Simplify.

$$316827x+39483y=237861,316827x-121401y=658329$$

Subtract 316827x-121401y=658329 from 316827x+39483y=237861 by subtracting like terms on each side of the equal sign.

$$316827x-316827x+39483y+121401y=237861-658329$$

Add 316827x to -316827x. Terms 316827x and -316827x cancel out, leaving an equation with only one variable that can be solved.

$$39483y+121401y=237861-658329$$

Add 39483y to 121401y.

$$160884y=237861-658329$$

Add 237861 to -658329.

$$160884y=-420468$$

Divide both sides by 160884.

$$y=-\frac{35039}{13407}$$

Substitute -\frac{35039}{13407} for y in 321x-123y=667. Because the resulting equation contains only one variable, you can solve for x directly.

$$321x-123\left(-\frac{35039}{13407}\right)=667$$

Multiply -123 times -\frac{35039}{13407}.

$$321x+\frac{35039}{109}=667$$

Subtract \frac{35039}{109} from both sides of the equation.

$$321x=\frac{37664}{109}$$

Divide both sides by 321.

$$x=\frac{352}{327}$$

The system is now solved.

$$x=\frac{352}{327},y=-\frac{35039}{13407}$$