Solve {l}{10x+5y=170}{6x+10y=200}

$$\left\{ \begin{array} { l } { 10 x + 5 y = 170 } \\ { 6 x + 10 y = 200 } \end{array} \right.$$

Solve for x, y

$$x=10$$

$$y=14$$

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.

$$10x+5y=170,6x+10y=200$$

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

$$10x+5y=170$$

Subtract 5y from both sides of the equation.

$$10x=-5y+170$$

Divide both sides by 10.

$$x=\frac{1}{10}\left(-5y+170\right)$$

Multiply \frac{1}{10} times -5y+170.

$$x=-\frac{1}{2}y+17$$

Substitute -\frac{y}{2}+17 for x in the other equation, 6x+10y=200.

$$6\left(-\frac{1}{2}y+17\right)+10y=200$$

Multiply 6 times -\frac{y}{2}+17.

$$-3y+102+10y=200$$

Add -3y to 10y.

$$7y+102=200$$

Subtract 102 from both sides of the equation.

$$7y=98$$

Divide both sides by 7.

$$y=14$$

Substitute 14 for y in x=-\frac{1}{2}y+17. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=-\frac{1}{2}\times 14+17$$

Multiply -\frac{1}{2} times 14.

$$x=-7+17$$

Add 17 to -7.

$$x=10$$

The system is now solved.

$$x=10,y=14$$

Steps Using Matrices

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

$$10x+5y=170,6x+10y=200$$

Write the equations in matrix form.

$$\left(\begin{matrix}10&5\\6&10\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}170\\200\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}10&5\\6&10\end{matrix}\right).

$$inverse(\left(\begin{matrix}10&5\\6&10\end{matrix}\right))\left(\begin{matrix}10&5\\6&10\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}10&5\\6&10\end{matrix}\right))\left(\begin{matrix}170\\200\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}10&5\\6&10\end{matrix}\right))\left(\begin{matrix}170\\200\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}10&5\\6&10\end{matrix}\right))\left(\begin{matrix}170\\200\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{10}{10\times 10-5\times 6}&-\frac{5}{10\times 10-5\times 6}\\-\frac{6}{10\times 10-5\times 6}&\frac{10}{10\times 10-5\times 6}\end{matrix}\right)\left(\begin{matrix}170\\200\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{1}{7}&-\frac{1}{14}\\-\frac{3}{35}&\frac{1}{7}\end{matrix}\right)\left(\begin{matrix}170\\200\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{1}{7}\times 170-\frac{1}{14}\times 200\\-\frac{3}{35}\times 170+\frac{1}{7}\times 200\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}10\\14\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=10,y=14$$

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.

$$10x+5y=170,6x+10y=200$$

To make 10x and 6x equal, multiply all terms on each side of the first equation by 6 and all terms on each side of the second by 10.

$$6\times 10x+6\times 5y=6\times 170,10\times 6x+10\times 10y=10\times 200$$

Simplify.

$$60x+30y=1020,60x+100y=2000$$

Subtract 60x+100y=2000 from 60x+30y=1020 by subtracting like terms on each side of the equal sign.

$$60x-60x+30y-100y=1020-2000$$

Add 60x to -60x. Terms 60x and -60x cancel out, leaving an equation with only one variable that can be solved.

$$30y-100y=1020-2000$$

Add 30y to -100y.

$$-70y=1020-2000$$

Add 1020 to -2000.

$$-70y=-980$$

Divide both sides by -70.

$$y=14$$

Substitute 14 for y in 6x+10y=200. Because the resulting equation contains only one variable, you can solve for x directly.

$$6x+10\times 14=200$$

Multiply 10 times 14.

$$6x+140=200$$

Subtract 140 from both sides of the equation.

$$6x=60$$

Divide both sides by 6.

$$x=10$$

The system is now solved.

$$x=10,y=14$$