Solve {l}{3x+2y=180}{5x+4y=620}

$$\left\{ \begin{array} { l } { 3 x + 2 y = 180 } \\ { 5 x + 4 y = 620 } \end{array} \right.$$

Solve for x, y

$$x=-260$$

$$y=480$$

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.

$$3x+2y=180,5x+4y=620$$

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

$$3x+2y=180$$

Subtract 2y from both sides of the equation.

$$3x=-2y+180$$

Divide both sides by 3.

$$x=\frac{1}{3}\left(-2y+180\right)$$

Multiply \frac{1}{3} times -2y+180.

$$x=-\frac{2}{3}y+60$$

Substitute -\frac{2y}{3}+60 for x in the other equation, 5x+4y=620.

$$5\left(-\frac{2}{3}y+60\right)+4y=620$$

Multiply 5 times -\frac{2y}{3}+60.

$$-\frac{10}{3}y+300+4y=620$$

Add -\frac{10y}{3} to 4y.

$$\frac{2}{3}y+300=620$$

Subtract 300 from both sides of the equation.

$$\frac{2}{3}y=320$$

Divide both sides of the equation by \frac{2}{3}, which is the same as multiplying both sides by the reciprocal of the fraction.

$$y=480$$

Substitute 480 for y in x=-\frac{2}{3}y+60. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=-\frac{2}{3}\times 480+60$$

Multiply -\frac{2}{3} times 480.

$$x=-320+60$$

Add 60 to -320.

$$x=-260$$

The system is now solved.

$$x=-260,y=480$$

Steps Using Matrices

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

$$3x+2y=180,5x+4y=620$$

Write the equations in matrix form.

$$\left(\begin{matrix}3&2\\5&4\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}180\\620\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}3&2\\5&4\end{matrix}\right).

$$inverse(\left(\begin{matrix}3&2\\5&4\end{matrix}\right))\left(\begin{matrix}3&2\\5&4\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}3&2\\5&4\end{matrix}\right))\left(\begin{matrix}180\\620\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}3&2\\5&4\end{matrix}\right))\left(\begin{matrix}180\\620\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}3&2\\5&4\end{matrix}\right))\left(\begin{matrix}180\\620\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{4}{3\times 4-2\times 5}&-\frac{2}{3\times 4-2\times 5}\\-\frac{5}{3\times 4-2\times 5}&\frac{3}{3\times 4-2\times 5}\end{matrix}\right)\left(\begin{matrix}180\\620\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}2&-1\\-\frac{5}{2}&\frac{3}{2}\end{matrix}\right)\left(\begin{matrix}180\\620\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}2\times 180-620\\-\frac{5}{2}\times 180+\frac{3}{2}\times 620\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-260\\480\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=-260,y=480$$

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.

$$3x+2y=180,5x+4y=620$$

To make 3x and 5x equal, multiply all terms on each side of the first equation by 5 and all terms on each side of the second by 3.

$$5\times 3x+5\times 2y=5\times 180,3\times 5x+3\times 4y=3\times 620$$

Simplify.

$$15x+10y=900,15x+12y=1860$$

Subtract 15x+12y=1860 from 15x+10y=900 by subtracting like terms on each side of the equal sign.

$$15x-15x+10y-12y=900-1860$$

Add 15x to -15x. Terms 15x and -15x cancel out, leaving an equation with only one variable that can be solved.

$$10y-12y=900-1860$$

Add 10y to -12y.

$$-2y=900-1860$$

Add 900 to -1860.

$$-2y=-960$$

Divide both sides by -2.

$$y=480$$

Substitute 480 for y in 5x+4y=620. Because the resulting equation contains only one variable, you can solve for x directly.

$$5x+4\times 480=620$$

Multiply 4 times 480.

$$5x+1920=620$$

Subtract 1920 from both sides of the equation.

$$5x=-1300$$

Divide both sides by 5.

$$x=-260$$

The system is now solved.

$$x=-260,y=480$$