Solve {l}{1/18(x+y)=1}{3/4x+7/9(x+y)=5}

$$\left\{ \begin{array} { l } { \frac { 1 } { 18 } ( x + y ) = 1 } \\ { \frac { 3 } { 4 } x + \frac { 7 } { 9 } ( x + y ) = 5 } \end{array} \right.$$

Solve for x, y

$$x=-12$$

$$y=30$$

Steps Using Substitution

Consider the first equation. Multiply both sides by 18, the reciprocal of \frac{1}{18}.

$$x+y=1\times 18$$

Multiply 1 and 18 to get 18.

$$x+y=18$$

Consider the second equation. Use the distributive property to multiply \frac{7}{9} by x+y.

$$\frac{3}{4}x+\frac{7}{9}x+\frac{7}{9}y=5$$

Combine \frac{3}{4}x and \frac{7}{9}x to get \frac{55}{36}x.

$$\frac{55}{36}x+\frac{7}{9}y=5$$

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.

$$x+y=18,\frac{55}{36}x+\frac{7}{9}y=5$$

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

$$x+y=18$$

Subtract y from both sides of the equation.

$$x=-y+18$$

Substitute -y+18 for x in the other equation, \frac{55}{36}x+\frac{7}{9}y=5.

$$\frac{55}{36}\left(-y+18\right)+\frac{7}{9}y=5$$

Multiply \frac{55}{36} times -y+18.

$$-\frac{55}{36}y+\frac{55}{2}+\frac{7}{9}y=5$$

Add -\frac{55y}{36} to \frac{7y}{9}.

$$-\frac{3}{4}y+\frac{55}{2}=5$$

Subtract \frac{55}{2} from both sides of the equation.

$$-\frac{3}{4}y=-\frac{45}{2}$$

Divide both sides of the equation by -\frac{3}{4}, which is the same as multiplying both sides by the reciprocal of the fraction.

$$y=30$$

Substitute 30 for y in x=-y+18. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=-30+18$$

Add 18 to -30.

$$x=-12$$

The system is now solved.

$$x=-12,y=30$$

Steps Using Matrices

Consider the first equation. Multiply both sides by 18, the reciprocal of \frac{1}{18}.

$$x+y=1\times 18$$

Multiply 1 and 18 to get 18.

$$x+y=18$$

Consider the second equation. Use the distributive property to multiply \frac{7}{9} by x+y.

$$\frac{3}{4}x+\frac{7}{9}x+\frac{7}{9}y=5$$

Combine \frac{3}{4}x and \frac{7}{9}x to get \frac{55}{36}x.

$$\frac{55}{36}x+\frac{7}{9}y=5$$

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

$$x+y=18,\frac{55}{36}x+\frac{7}{9}y=5$$

Write the equations in matrix form.

$$\left(\begin{matrix}1&1\\\frac{55}{36}&\frac{7}{9}\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}18\\5\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}1&1\\\frac{55}{36}&\frac{7}{9}\end{matrix}\right).

$$inverse(\left(\begin{matrix}1&1\\\frac{55}{36}&\frac{7}{9}\end{matrix}\right))\left(\begin{matrix}1&1\\\frac{55}{36}&\frac{7}{9}\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&1\\\frac{55}{36}&\frac{7}{9}\end{matrix}\right))\left(\begin{matrix}18\\5\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&1\\\frac{55}{36}&\frac{7}{9}\end{matrix}\right))\left(\begin{matrix}18\\5\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&1\\\frac{55}{36}&\frac{7}{9}\end{matrix}\right))\left(\begin{matrix}18\\5\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{\frac{7}{9}}{\frac{7}{9}-\frac{55}{36}}&-\frac{1}{\frac{7}{9}-\frac{55}{36}}\\-\frac{\frac{55}{36}}{\frac{7}{9}-\frac{55}{36}}&\frac{1}{\frac{7}{9}-\frac{55}{36}}\end{matrix}\right)\left(\begin{matrix}18\\5\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{28}{27}&\frac{4}{3}\\\frac{55}{27}&-\frac{4}{3}\end{matrix}\right)\left(\begin{matrix}18\\5\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{28}{27}\times 18+\frac{4}{3}\times 5\\\frac{55}{27}\times 18-\frac{4}{3}\times 5\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-12\\30\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=-12,y=30$$

Steps Using Elimination

Consider the first equation. Multiply both sides by 18, the reciprocal of \frac{1}{18}.

$$x+y=1\times 18$$

Multiply 1 and 18 to get 18.

$$x+y=18$$

Consider the second equation. Use the distributive property to multiply \frac{7}{9} by x+y.

$$\frac{3}{4}x+\frac{7}{9}x+\frac{7}{9}y=5$$

Combine \frac{3}{4}x and \frac{7}{9}x to get \frac{55}{36}x.

$$\frac{55}{36}x+\frac{7}{9}y=5$$

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.

$$x+y=18,\frac{55}{36}x+\frac{7}{9}y=5$$

To make x and \frac{55x}{36} equal, multiply all terms on each side of the first equation by \frac{55}{36} and all terms on each side of the second by 1.

$$\frac{55}{36}x+\frac{55}{36}y=\frac{55}{36}\times 18,\frac{55}{36}x+\frac{7}{9}y=5$$

Simplify.

$$\frac{55}{36}x+\frac{55}{36}y=\frac{55}{2},\frac{55}{36}x+\frac{7}{9}y=5$$

Subtract \frac{55}{36}x+\frac{7}{9}y=5 from \frac{55}{36}x+\frac{55}{36}y=\frac{55}{2} by subtracting like terms on each side of the equal sign.

$$\frac{55}{36}x-\frac{55}{36}x+\frac{55}{36}y-\frac{7}{9}y=\frac{55}{2}-5$$

Add \frac{55x}{36} to -\frac{55x}{36}. Terms \frac{55x}{36} and -\frac{55x}{36} cancel out, leaving an equation with only one variable that can be solved.

$$\frac{55}{36}y-\frac{7}{9}y=\frac{55}{2}-5$$

Add \frac{55y}{36} to -\frac{7y}{9}.

$$\frac{3}{4}y=\frac{55}{2}-5$$

Add \frac{55}{2} to -5.

$$\frac{3}{4}y=\frac{45}{2}$$

Divide both sides of the equation by \frac{3}{4}, which is the same as multiplying both sides by the reciprocal of the fraction.

$$y=30$$

Substitute 30 for y in \frac{55}{36}x+\frac{7}{9}y=5. Because the resulting equation contains only one variable, you can solve for x directly.

$$\frac{55}{36}x+\frac{7}{9}\times 30=5$$

Multiply \frac{7}{9} times 30.

$$\frac{55}{36}x+\frac{70}{3}=5$$

Subtract \frac{70}{3} from both sides of the equation.

$$\frac{55}{36}x=-\frac{55}{3}$$

Divide both sides of the equation by \frac{55}{36}, which is the same as multiplying both sides by the reciprocal of the fraction.

$$x=-12$$

The system is now solved.

$$x=-12,y=30$$