Solve {l}{x+y=5}{200x=300y}

$$\left\{ \begin{array} { l } { x + y = 5 } \\ { 200 x = 300 y } \end{array} \right.$$

Solve for x, y

$$x=3$$

$$y=2$$

Steps Using Substitution

Consider the second equation. Subtract 300y from both sides.

$$200x-300y=0$$

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.

$$x+y=5,200x-300y=0$$

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

$$x+y=5$$

Subtract y from both sides of the equation.

$$x=-y+5$$

Substitute -y+5 for x in the other equation, 200x-300y=0.

$$200\left(-y+5\right)-300y=0$$

Multiply 200 times -y+5.

$$-200y+1000-300y=0$$

Add -200y to -300y.

$$-500y+1000=0$$

Subtract 1000 from both sides of the equation.

$$-500y=-1000$$

Divide both sides by -500.

$$y=2$$

Substitute 2 for y in x=-y+5. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=-2+5$$

Add 5 to -2.

$$x=3$$

The system is now solved.

$$x=3,y=2$$

Steps Using Matrices

Consider the second equation. Subtract 300y from both sides.

$$200x-300y=0$$

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

$$x+y=5,200x-300y=0$$

Write the equations in matrix form.

$$\left(\begin{matrix}1&1\\200&-300\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}5\\0\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}1&1\\200&-300\end{matrix}\right).

$$inverse(\left(\begin{matrix}1&1\\200&-300\end{matrix}\right))\left(\begin{matrix}1&1\\200&-300\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&1\\200&-300\end{matrix}\right))\left(\begin{matrix}5\\0\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&1\\200&-300\end{matrix}\right))\left(\begin{matrix}5\\0\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&1\\200&-300\end{matrix}\right))\left(\begin{matrix}5\\0\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{300}{-300-200}&-\frac{1}{-300-200}\\-\frac{200}{-300-200}&\frac{1}{-300-200}\end{matrix}\right)\left(\begin{matrix}5\\0\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{3}{5}&\frac{1}{500}\\\frac{2}{5}&-\frac{1}{500}\end{matrix}\right)\left(\begin{matrix}5\\0\end{matrix}\right)$$

Multiply the matrices.

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

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}3\\2\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=3,y=2$$

Steps Using Elimination

Consider the second equation. Subtract 300y from both sides.

$$200x-300y=0$$

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.

$$x+y=5,200x-300y=0$$

To make x and 200x equal, multiply all terms on each side of the first equation by 200 and all terms on each side of the second by 1.

$$200x+200y=200\times 5,200x-300y=0$$

Simplify.

$$200x+200y=1000,200x-300y=0$$

Subtract 200x-300y=0 from 200x+200y=1000 by subtracting like terms on each side of the equal sign.

$$200x-200x+200y+300y=1000$$

Add 200x to -200x. Terms 200x and -200x cancel out, leaving an equation with only one variable that can be solved.

$$200y+300y=1000$$

Add 200y to 300y.

$$500y=1000$$

Divide both sides by 500.

$$y=2$$

Substitute 2 for y in 200x-300y=0. Because the resulting equation contains only one variable, you can solve for x directly.

$$200x-300\times 2=0$$

Multiply -300 times 2.

$$200x-600=0$$

Add 600 to both sides of the equation.

$$200x=600$$

Divide both sides by 200.

$$x=3$$

The system is now solved.

$$x=3,y=2$$