Solve {l}{3x+4y=100}{9y+10x=30}

$$\left. \begin{array} { l } { 3 x + 4 y = 100 } \\ { 9 y + 10 x = 30 } \end{array} \right.$$

Solve for x, y

$$x=-60$$

$$y=70$$

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.

$$3x+4y=100,10x+9y=30$$

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

$$3x+4y=100$$

Subtract 4y from both sides of the equation.

$$3x=-4y+100$$

Divide both sides by 3.

$$x=\frac{1}{3}\left(-4y+100\right)$$

Multiply \frac{1}{3} times -4y+100.

$$x=-\frac{4}{3}y+\frac{100}{3}$$

Substitute \frac{-4y+100}{3} for x in the other equation, 10x+9y=30.

$$10\left(-\frac{4}{3}y+\frac{100}{3}\right)+9y=30$$

Multiply 10 times \frac{-4y+100}{3}.

$$-\frac{40}{3}y+\frac{1000}{3}+9y=30$$

Add -\frac{40y}{3} to 9y.

$$-\frac{13}{3}y+\frac{1000}{3}=30$$

Subtract \frac{1000}{3} from both sides of the equation.

$$-\frac{13}{3}y=-\frac{910}{3}$$

Divide both sides of the equation by -\frac{13}{3}, which is the same as multiplying both sides by the reciprocal of the fraction.

$$y=70$$

Substitute 70 for y in x=-\frac{4}{3}y+\frac{100}{3}. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=-\frac{4}{3}\times 70+\frac{100}{3}$$

Multiply -\frac{4}{3} times 70.

$$x=\frac{-280+100}{3}$$

Add \frac{100}{3} to -\frac{280}{3} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x=-60$$

The system is now solved.

$$x=-60,y=70$$

Steps Using Matrices

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

$$3x+4y=100,10x+9y=30$$

Write the equations in matrix form.

$$\left(\begin{matrix}3&4\\10&9\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}100\\30\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}3&4\\10&9\end{matrix}\right).

$$inverse(\left(\begin{matrix}3&4\\10&9\end{matrix}\right))\left(\begin{matrix}3&4\\10&9\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}3&4\\10&9\end{matrix}\right))\left(\begin{matrix}100\\30\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}3&4\\10&9\end{matrix}\right))\left(\begin{matrix}100\\30\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}3&4\\10&9\end{matrix}\right))\left(\begin{matrix}100\\30\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{9}{3\times 9-4\times 10}&-\frac{4}{3\times 9-4\times 10}\\-\frac{10}{3\times 9-4\times 10}&\frac{3}{3\times 9-4\times 10}\end{matrix}\right)\left(\begin{matrix}100\\30\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{9}{13}&\frac{4}{13}\\\frac{10}{13}&-\frac{3}{13}\end{matrix}\right)\left(\begin{matrix}100\\30\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{9}{13}\times 100+\frac{4}{13}\times 30\\\frac{10}{13}\times 100-\frac{3}{13}\times 30\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-60\\70\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=-60,y=70$$

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.

$$3x+4y=100,10x+9y=30$$

To make 3x and 10x equal, multiply all terms on each side of the first equation by 10 and all terms on each side of the second by 3.

$$10\times 3x+10\times 4y=10\times 100,3\times 10x+3\times 9y=3\times 30$$

Simplify.

$$30x+40y=1000,30x+27y=90$$

Subtract 30x+27y=90 from 30x+40y=1000 by subtracting like terms on each side of the equal sign.

$$30x-30x+40y-27y=1000-90$$

Add 30x to -30x. Terms 30x and -30x cancel out, leaving an equation with only one variable that can be solved.

$$40y-27y=1000-90$$

Add 40y to -27y.

$$13y=1000-90$$

Add 1000 to -90.

$$13y=910$$

Divide both sides by 13.

$$y=70$$

Substitute 70 for y in 10x+9y=30. Because the resulting equation contains only one variable, you can solve for x directly.

$$10x+9\times 70=30$$

Multiply 9 times 70.

$$10x+630=30$$

Subtract 630 from both sides of the equation.

$$10x=-600$$

Divide both sides by 10.

$$x=-60$$

The system is now solved.

$$x=-60,y=70$$